diff --git a/core/database.class.php b/core/database.class.php
index 0d19c9e0..f0480699 100644
--- a/core/database.class.php
+++ b/core/database.class.php
@@ -20,7 +20,7 @@ class Querylet {
 	 * @param \Querylet $querylet
 	 */
 	public function append($querylet) {
-		assert(!is_null($querylet));
+		assert('!is_null($querylet)');
 		$this->sql .= $querylet->sql;
 		$this->variables = array_merge($this->variables, $querylet->variables);
 	}
@@ -295,7 +295,7 @@ class MemcacheCache implements CacheEngine {
 	 * @return array|bool|string
 	 */
 	public function get($key) {
-		assert(!is_null($key));
+		assert('!is_null($key)');
 		$val = $this->memcache->get($key);
 		if((DEBUG_CACHE === true) || (is_null(DEBUG_CACHE) && @$_GET['DEBUG_CACHE'])) {
 			$hit = $val === false ? "miss" : "hit";
@@ -317,7 +317,7 @@ class MemcacheCache implements CacheEngine {
 	 * @param int $time
 	 */
 	public function set($key, $val, $time=0) {
-		assert(!is_null($key));
+		assert('!is_null($key)');
 		$this->memcache->set($key, $val, false, $time);
 		if((DEBUG_CACHE === true) || (is_null(DEBUG_CACHE) && @$_GET['DEBUG_CACHE'])) {
 			file_put_contents("data/cache.log", "Cache set: $key ($time)\n", FILE_APPEND);
@@ -328,7 +328,7 @@ class MemcacheCache implements CacheEngine {
 	 * @param string $key
 	 */
 	public function delete($key) {
-		assert(!is_null($key));
+		assert('!is_null($key)');
 		$this->memcache->delete($key);
 		if((DEBUG_CACHE === true) || (is_null(DEBUG_CACHE) && @$_GET['DEBUG_CACHE'])) {
 			file_put_contents("data/cache.log", "Cache delete: $key\n", FILE_APPEND);
@@ -354,7 +354,7 @@ class APCCache implements CacheEngine {
 	}
 
 	public function get($key) {
-		assert(!is_null($key));
+		assert('!is_null($key)');
 		$val = apc_fetch($key);
 		if($val) {
 			$this->hits++;
@@ -367,12 +367,12 @@ class APCCache implements CacheEngine {
 	}
 
 	public function set($key, $val, $time=0) {
-		assert(!is_null($key));
+		assert('!is_null($key)');
 		apc_store($key, $val, $time);
 	}
 
 	public function delete($key) {
-		assert(!is_null($key));
+		assert('!is_null($key)');
 		apc_delete($key);
 	}
 
diff --git a/core/exceptions.class.php b/core/exceptions.class.php
index 14765760..d2400893 100644
--- a/core/exceptions.class.php
+++ b/core/exceptions.class.php
@@ -22,3 +22,8 @@ class PermissionDeniedException extends SCoreException {}
  * Example: Image::by_id(-1) returns null
  */
 class ImageDoesNotExist extends SCoreException {}
+
+/*
+ * For validate_input()
+ */
+class InvalidInput extends SCoreException {}
diff --git a/core/imageboard.pack.php b/core/imageboard.pack.php
index ad5de8d2..2c914536 100644
--- a/core/imageboard.pack.php
+++ b/core/imageboard.pack.php
@@ -97,7 +97,7 @@ class Image {
 	 * @return Image
 	 */
 	public static function by_id(/*int*/ $id) {
-		assert(is_numeric($id));
+		assert('is_numeric($id)');
 		global $database;
 		$row = $database->get_row("SELECT * FROM images WHERE images.id=:id", array("id"=>$id));
 		return ($row ? new Image($row) : null);
@@ -110,7 +110,7 @@ class Image {
 	 * @return Image
 	 */
 	public static function by_hash(/*string*/ $hash) {
-		assert(is_string($hash));
+		assert('is_string($hash)');
 		global $database;
 		$row = $database->get_row("SELECT images.* FROM images WHERE hash=:hash", array("hash"=>$hash));
 		return ($row ? new Image($row) : null);
@@ -123,7 +123,7 @@ class Image {
 	 * @return Image
 	 */
 	public static function by_random($tags=array()) {
-		assert(is_array($tags));
+		assert('is_array($tags)');
 		$max = Image::count_images($tags);
 		if ($max < 1) return null;		// From Issue #22 - opened by HungryFeline on May 30, 2011.
 		$rand = mt_rand(0, $max-1);
@@ -142,9 +142,9 @@ class Image {
 	 * @return Image[]
 	 */
 	public static function find_images(/*int*/ $start, /*int*/ $limit, $tags=array()) {
-		assert(is_numeric($start));
-		assert(is_numeric($limit));
-		assert(is_array($tags));
+		assert('is_numeric($start)');
+		assert('is_numeric($limit)');
+		assert('is_array($tags)');
 		global $database, $user, $config, $order_sql;
 
 		$images = array();
@@ -246,7 +246,7 @@ class Image {
 	 * @return mixed
 	 */
 	public static function count_images($tags=array()) {
-		assert(is_array($tags));
+		assert('is_array($tags)');
 		global $database;
 		$tag_count = count($tags);
 
@@ -278,7 +278,7 @@ class Image {
 	 * @return float
 	 */
 	public static function count_pages($tags=array()) {
-		assert(is_array($tags));
+		assert('is_array($tags)');
 		global $config;
 		return ceil(Image::count_images($tags) / $config->get_int('index_images'));
 	}
@@ -299,8 +299,8 @@ class Image {
 	 * @return Image
 	 */
 	public function get_next($tags=array(), $next=true) {
-		assert(is_array($tags));
-		assert(is_bool($next));
+		assert('is_array($tags)');
+		assert('is_bool($next)');
 		global $database;
 
 		if($next) {
@@ -562,18 +562,17 @@ class Image {
 	 * @param string[] $tags
 	 */
 	public function set_tags($tags) {
+		assert('is_array($tags) && count($tags) > 0', var_export($tags, true));
 		global $database;
 
-		assert(is_array($tags));
-
 		$tags = array_map(array('Tag', 'sanitise'), $tags);
 		$tags = Tag::resolve_aliases($tags);
 
-		assert(is_array($tags));
-		assert(count($tags) > 0);
-		$new_tags = implode(" ", $tags);
+		if(count($tags) <= 0) {
+			throw new SCoreException('Tried to set zero tags');
+		}
 
-		if($new_tags != $this->get_tag_list()) {
+		if(implode(" ", $tags) != $this->get_tag_list()) {
 			// delete old
 			$this->delete_tags_from_image();
 			// insert each new tags
@@ -726,7 +725,7 @@ class Image {
 	 * @return \Querylet
 	 */
 	private static function build_search_querylet($terms) {
-		assert(is_array($terms));
+		assert('is_array($terms)');
 		global $database;
 		if($database->get_driver_name() === "mysql")
 			return Image::build_ugly_search_querylet($terms);
@@ -1104,7 +1103,7 @@ class Tag {
 	 * @return mixed
 	 */
 	public static function sanitise($tag) {
-		assert(is_string($tag));
+		assert('is_string($tag)');
 		$tag = preg_replace("/[\s?*]/", "", $tag);            # whitespace
 		$tag = preg_replace('/\x20(\x0e|\x0f)/', '', $tag);   # unicode RTL
 		$tag = preg_replace("/\.+/", ".", $tag);              # strings of dots?
@@ -1120,7 +1119,7 @@ class Tag {
 	 * @return array
 	 */
 	public static function explode($tags, $tagme=true) {
-		assert(is_string($tags) || is_array($tags));
+		assert('is_string($tags) || is_array($tags)');
 
 		if(is_string($tags)) {
 			$tags = explode(' ', trim($tags));
@@ -1151,7 +1150,7 @@ class Tag {
 	 * @return string
 	 */
 	public static function implode($tags) {
-		assert(is_string($tags) || is_array($tags));
+		assert('is_string($tags) || is_array($tags)');
 
 		if(is_array($tags)) {
 			sort($tags);
@@ -1169,7 +1168,7 @@ class Tag {
 	 * @return string
 	 */
 	public static function resolve_alias($tag) {
-		assert(is_string($tag));
+		assert('is_string($tag)');
 
 		$negative = false;
 		if(!empty($tag) && ($tag[0] == '-')) {
@@ -1224,7 +1223,7 @@ class Tag {
 	 * @return array
 	 */
 	public static function resolve_aliases($tags) {
-		assert(is_array($tags));
+		assert('is_array($tags)');
 
 		$new = array();
 
diff --git a/core/user.class.php b/core/user.class.php
index 679e0fa6..34d111aa 100644
--- a/core/user.class.php
+++ b/core/user.class.php
@@ -94,7 +94,7 @@ class User {
 	 * @return null|User
 	 */
 	public static function by_id(/*int*/ $id) {
-		assert(is_numeric($id));
+		assert('is_numeric($id)', var_export($id, true));
 		global $database;
 		if($id === 1) {
 			$cached = $database->cache->get('user-id:'.$id);
@@ -111,7 +111,7 @@ class User {
 	 * @return null|User
 	 */
 	public static function by_name(/*string*/ $name) {
-		assert(is_string($name));
+		assert('is_string($name)', var_export($name, true));
 		global $database;
 		$row = $database->get_row($database->scoreql_to_sql("SELECT * FROM users WHERE SCORE_STRNORM(name) = SCORE_STRNORM(:name)"), array("name"=>$name));
 		return is_null($row) ? null : new User($row);
@@ -124,8 +124,8 @@ class User {
 	 * @return null|User
 	 */
 	public static function by_name_and_pass(/*string*/ $name, /*string*/ $pass) {
-		assert(is_string($name));
-		assert(is_string($pass));
+		assert('is_string($name)', var_export($name, true));
+		assert('is_string($pass)', var_export($pass, true));
 		$user = User::by_name($name);
 		if($user) {
 			if($user->passhash == md5(strtolower($name) . $pass)) {
@@ -143,8 +143,8 @@ class User {
 	 * @return array
 	 */
 	public static function by_list(/*int*/ $offset, /*int*/ $limit=50) {
-		assert(is_numeric($offset));
-		assert(is_numeric($limit));
+		assert('is_numeric($offset)', var_export($offset, true));
+		assert('is_numeric($limit)', var_export($limit, true));
 		global $database;
 		$rows = $database->get_all("SELECT * FROM users WHERE id >= :start AND id < :end", array("start"=>$offset, "end"=>$offset+$limit));
 		return array_map("_new_user", $rows);
@@ -196,7 +196,7 @@ class User {
 	 * @param string $class
 	 */
 	public function set_class(/*string*/ $class) {
-		assert(is_string($class));
+		assert('is_string($class)', var_export($class, true));
 		global $database;
 		$database->Execute("UPDATE users SET class=:class WHERE id=:id", array("class"=>$class, "id"=>$this->id));
 		log_info("core-user", 'Set class for '.$this->name.' to '.$class);
diff --git a/core/util.inc.php b/core/util.inc.php
index 603facf4..95eb318b 100644
--- a/core/util.inc.php
+++ b/core/util.inc.php
@@ -263,6 +263,64 @@ function isValidDate($date) {
 	return false;
 }
 
+function validate_input($inputs) {
+	$outputs = array();
+
+	foreach($inputs as $key => $validations) {
+		$flags = explode(',', $validations);
+		if(in_array('optional', $flags)) {
+			if(!isset($_POST[$key])) {
+				continue;
+			}
+		}
+
+		if(!isset($_POST[$key])) {
+			throw new InvalidInput("Input '$key' not set");
+		}
+
+		$value = $_POST[$key];
+
+		if(in_array('user_id', $flags)) {
+			$id = int_escape($value);
+			if(in_array('exists', $flags)) {
+				if(is_null(User::by_id($id))) {
+					throw new InvalidInput("User #$id does not exist");
+				}
+			}
+			$outputs[$key] = $id;
+		}
+		else if(in_array('user_name', $flags)) {
+			if(strlen($value) < 1) {
+				throw new InvalidInput("Username must be at least 1 character");
+			}
+			else if(!preg_match('/^[a-zA-Z0-9-_]+$/', $value)) {
+				throw new InvalidInput(
+						"Username contains invalid characters. Allowed characters are ".
+						"letters, numbers, dash, and underscore");
+			}
+			$outputs[$key] = $value;
+		}
+		else if(in_array('user_class', $flags)) {
+			global $_user_classes;
+			if(!array_key_exists($value, $_user_classes)) {
+				throw new InvalidInput("Invalid user class: ".html_escape($class));
+			}
+			$outputs[$key] = $value;
+		}
+		else if(in_array('email', $flags)) {
+			$outputs[$key] = $value;
+		}
+		else if(in_array('password', $flags)) {
+			$outputs[$key] = $value;
+		}
+		else {
+			throw new InvalidInput("Unknown validation '$validations'");
+		}
+	}
+
+	return $outputs;
+}
+
 /**
  * Give a HTML string which shows an IP (if the user is allowed to see IPs),
  * and a link to ban that IP (if the user is allowed to ban IPs)
@@ -1439,6 +1497,19 @@ function get_debug_info() {
 	return $debug;
 }
 
+function score_assert_handler($file, $line, $code, $desc = null) {
+	$file = basename($file);
+	print("Assertion failed at $file:$line: $code ($desc)");
+	/*
+	print("<pre>");
+	debug_print_backtrace();
+	print("</pre>");
+	*/
+}
+//assert_options(ASSERT_ACTIVE, 1);
+assert_options(ASSERT_WARNING, 0);
+assert_options(ASSERT_QUIET_EVAL, 1);
+assert_options(ASSERT_CALLBACK, 'score_assert_handler');
 
 /* * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * *\
 * Request initialisation stuff                                              *
diff --git a/ext/user/main.php b/ext/user/main.php
index e5791f4d..a0d87426 100644
--- a/ext/user/main.php
+++ b/ext/user/main.php
@@ -174,7 +174,7 @@ class UserPage extends Extension {
 				log_info("user", "Logged out");
 				$page->set_mode("redirect");
                                 
-                                // Try forwarding to same page on logout unless user comes from registration page
+				// Try forwarding to same page on logout unless user comes from registration page
 				if ($config->get_int("user_loginshowprofile",0) == 0 && 
 							isset($_SERVER['HTTP_REFERER']) &&
 							strstr($_SERVER['HTTP_REFERER'], "post/"))
@@ -190,49 +190,37 @@ class UserPage extends Extension {
 			}
 
 			else if($event->get_arg(0) == "change_name") {
-				if(isset($_POST['id']) && isset($_POST['name'])) {
-					$duser = User::by_id($_POST['id']);
-					if ( ! $duser instanceof User) {
-						throw new NullUserException("Error: the user id does not exist!");
-					}
-					$name = $_POST['name'];
-					$this->change_name_wrapper($duser, $name);
-				}
+				$input = validate_input(array(
+					'id' => 'user_id,exists',
+					'name' => 'user_name',
+				));
+				$duser = User::by_id($input['id']);
+				$this->change_name_wrapper($duser, $input['name']);
 			}
 			else if($event->get_arg(0) == "change_pass") {
-				if(isset($_POST['id']) && isset($_POST['pass1']) && isset($_POST['pass2'])) {
-					$duser = User::by_id($_POST['id']);
-					if ( ! $duser instanceof User) {
-						throw new NullUserException("Error: the user id does not exist!");
-					}
-					$pass1 = $_POST['pass1'];
-					$pass2 = $_POST['pass2'];
-					$this->change_password_wrapper($duser, $pass1, $pass2);
-				}
+				$input = validate_input(array(
+					'id' => 'user_id,exists',
+					'pass1' => 'password',
+					'pass2' => 'password',
+				));
+				$duser = User::by_id($input['id']);
+				$this->change_password_wrapper($duser, $input['pass1'], $input['pass2']);
 			}
 			else if($event->get_arg(0) == "change_email") {
-				if(isset($_POST['id']) && isset($_POST['address'])) {
-					$duser = User::by_id($_POST['id']);
-					if ( ! $duser instanceof User) {
-						throw new NullUserException("Error: the user id does not exist!");
-					}
-					$address = $_POST['address'];
-					$this->change_email_wrapper($duser, $address);
-				}
+				$input = validate_input(array(
+					'id' => 'user_id,exists',
+					'address' => 'email',
+				));
+				$duser = User::by_id($input['id']);
+				$this->change_email_wrapper($duser, $input['address']);
 			}
 			else if($event->get_arg(0) == "change_class") {
-				global $_user_classes;
-				if(isset($_POST['id']) && isset($_POST['class'])) {
-					$duser = User::by_id($_POST['id']);
-					if ( ! $duser instanceof User) {
-						throw new NullUserException("Error: the user id does not exist!");
-					}
-					$class = $_POST['class'];
-					if(!array_key_exists($class, $_user_classes)) {
-						throw Exception("Invalid user class: ".html_escape($class));
-					}
-					$this->change_class_wrapper($duser, $class);
-				}
+				$input = validate_input(array(
+					'id' => 'user_id,exists',
+					'class' => 'user_class',
+				));
+				$duser = User::by_id($input['id']);
+				$this->change_class_wrapper($duser, $input['class']);
 			}
 			else if($event->get_arg(0) == "delete_user") {
 				$this->delete_user($page, isset($_POST["with_images"]), isset($_POST["with_comments"]));
@@ -459,7 +447,7 @@ class UserPage extends Extension {
 					"Username contains invalid characters. Allowed characters are ".
 					"letters, numbers, dash, and underscore");
 		}
-		else if($database->get_row($database->scoreql_to_sql("SELECT * FROM users WHERE SCORE_STRNORM(name) = SCORE_STRNORM(:name)"), array("name"=>$name))) {
+		else if(User::by_name($name)) {
 			throw new UserCreationException("That username is already taken");
 		}
 	}
@@ -601,12 +589,7 @@ class UserPage extends Extension {
 		global $user;
 
 		if($user->class->name == "admin") {
-			$duser = User::by_id($_POST['id']);
-			if ( ! $duser instanceof User) {
-				throw new NullUserException("Error: the user id does not exist!");
-			}
 			$duser->set_class($class);
-
 			flash_message("Class changed");
 			$this->redirect_to_user($duser);
 		}