PHP is_serialized() function.

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.
 */ 

/**
 * Thanks to Chris for great solution, i have add only small changes
 */

/**
 * 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>, Frank Bültge <frank@bueltge.de>
 * @copyright	Copyright (c) 2009 Chris Smith (http://www.cs278.org/), 2011 Frank Bültge (http://bueltge.de)
 * @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 ( 'b:0;' === $value ) {
		$result = FALSE;
		return TRUE;
	}

	$length	= strlen($value);
	$end	= '';
	
	if ( isset( $value[0] ) ) {
		switch ($value[0]) {
			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;
	
				switch ( $value[2] ) {
					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;
}