is_serialized.php

<?php
/**
 * This program is free software. It comes without any warranty, to
 * the extent permitted by applicable law. You can redistribute it
 * and/or modify it under the terms of the Do What The Fuck You Want
 * To Public License, Version 2, as published by Sam Hocevar. See
 * http://sam.zoy.org/wtfpl/COPYING for more details.
 */ 

/**
 * Tests if an input is valid PHP serialized string.
 *
 * Checks if a string is serialized using quick string manipulation
 * to throw out obviously incorrect strings. Unserialize is then run
 * on the string to perform the final verification.
 *
 * Valid serialized forms are the following:
 * <ul>
 * <li>boolean: <code>b:1;</code></li>
 * <li>integer: <code>i:1;</code></li>
 * <li>double: <code>d:0.2;</code></li>
 * <li>string: <code>s:4:"test";</code></li>
 * <li>array: <code>a:3:{i:0;i:1;i:1;i:2;i:2;i:3;}</code></li>
 * <li>object: <code>O:8:"stdClass":0:{}</code></li>
 * <li>null: <code>N;</code></li>
 * </ul>
 *
 * @author		Chris Smith <code+php@chris.cs278.org>
 * @copyright	Copyright (c) 2009 Chris Smith (http://www.cs278.org/)
 * @license		http://sam.zoy.org/wtfpl/ WTFPL
 * @param		string	$value	Value to test for serialized form
 * @param		mixed	$result	Result of unserialize() of the $value
 * @return		boolean			True if $value is serialized data, otherwise false
 */
function is_serialized($value, &$result = null)
{
	// Bit of a give away this one
	if (!is_string($value))
	{
		return false;
	}

	// Serialized false, return true. unserialize() returns false on an
	// invalid string or it could return false if the string is serialized
	// false, eliminate that possibility.
	if ($value === 'b:0;')
	{
		$result = false;
		return true;
	}

	$length	= strlen($value);
	$end	= '';

	$value0 = $value[0];
	switch ($value0)
	{
		case 's':
			if ($value[$length - 2] !== '"')
			{
				return false;
			}
		case 'b':
		case 'i':
		case 'd':
			// This looks odd but it is quicker than isset()ing
			$end .= ';';
		case 'a':
		case 'O':
			$end .= '}';

			if ($value[1] !== ':')
			{
				return false;
			}

			$value2 = $value[2];
			switch ($value2)
			{
				case 0:
				case 1:
				case 2:
				case 3:
				case 4:
				case 5:
				case 6:
				case 7:
				case 8:
				case 9:
				break;

				default:
					return false;
			}
		case 'N':
			$end .= ';';

			if ($value[$length - 1] !== $end[0])
			{
				return false;
			}
		break;

		default:
			return false;
	}

	if (($result = @unserialize($value)) === false)
	{
		$result = null;
		return false;
	}
	return true;
}

Added some variables to spare the execution per each case in switch() comparison.