Check to see if a string is serialized?

From WordPress core functions:

<?php
function is_serialized( $data, $strict = true ) {
    // If it isn't a string, it isn't serialized.
    if ( ! is_string( $data ) ) {
        return false;
    }
    $data = trim( $data );
    if ( 'N;' === $data ) {
        return true;
    }
    if ( strlen( $data ) < 4 ) {
        return false;
    }
    if ( ':' !== $data[1] ) {
        return false;
    }
    if ( $strict ) {
        $lastc = substr( $data, -1 );
        if ( ';' !== $lastc && '}' !== $lastc ) {
            return false;
        }
    } else {
        $semicolon = strpos( $data, ';' );
        $brace     = strpos( $data, '}' );
        // Either ; or } must exist.
        if ( false === $semicolon && false === $brace ) {
            return false;
        }
        // But neither must be in the first X characters.
        if ( false !== $semicolon && $semicolon < 3 ) {
            return false;
        }
        if ( false !== $brace && $brace < 4 ) {
            return false;
        }
    }
    $token = $data[0];
    switch ( $token ) {
        case 's':
            if ( $strict ) {
                if ( '"' !== substr( $data, -2, 1 ) ) {
                    return false;
                }
            } elseif ( false === strpos( $data, '"' ) ) {
                return false;
            }
            // Or else fall through.
        case 'a':
        case 'O':
            return (bool) preg_match( "/^{$token}:[0-9]+:/s", $data );
        case 'b':
        case 'i':
        case 'd':
            $end = $strict ? '$' : '';
            return (bool) preg_match( "/^{$token}:[0-9.E+-]+;$end/", $data );
    }
    return false;
} 

Optimizing Pascal MARTIN's response

/**
 * Check if a string is serialized
 * @param string $string
 */
public static function is_serial($string) {
    return (@unserialize($string) !== false);
}

I would just try to unserialize it. This is how i would solve it

public static function is_serialized($string)
{
    try {
        unserialize($string);
    } catch (\Exception $e) {
        return false;
    }

    return true;
}

Or more like a helper function

function is_serialized($string) {
  try {
        unserialize($string);
    } catch (\Exception $e) {
        return false;
    }

    return true;
}

• The mentionned WordPress function does not really detect arrays (a:1:{42} is considered to be serialized) and falsely returns true on escaped strings like a:1:{s:3:\"foo\";s:3:\"bar\";} (although unserialize does not work)

• If you use the @unserialize way on the other side WordPress for example adds an ugly margin at the top of the backend when using define('WP_DEBUG', true);

• A working solution that solves both problems and circumvents the stfu-operator is:

function __is_serialized($var)
{
    if (!is_string($var) || $var == '') {
        return false;
    }
    set_error_handler(function ($errno, $errstr) {});
    $unserialized = unserialize($var);
    restore_error_handler();
    if ($var !== 'b:0;' && $unserialized === false) {
        return false;
    }
    return true;
}

see the wordpress function is_serialized

function is_serialized( $data, $strict = true ) {
// If it isn't a string, it isn't serialized.
if ( ! is_string( $data ) ) {
    return false;
}
$data = trim( $data );
if ( 'N;' === $data ) {
    return true;
}
if ( strlen( $data ) < 4 ) {
    return false;
}
if ( ':' !== $data[1] ) {
    return false;
}
if ( $strict ) {
    $lastc = substr( $data, -1 );
    if ( ';' !== $lastc && '}' !== $lastc ) {
        return false;
    }
} else {
    $semicolon = strpos( $data, ';' );
    $brace     = strpos( $data, '}' );
    // Either ; or } must exist.
    if ( false === $semicolon && false === $brace ) {
        return false;
    }
    // But neither must be in the first X characters.
    if ( false !== $semicolon && $semicolon < 3 ) {
        return false;
    }
    if ( false !== $brace && $brace < 4 ) {
        return false;
    }
}
$token = $data[0];
switch ( $token ) {
    case 's':
        if ( $strict ) {
            if ( '"' !== substr( $data, -2, 1 ) ) {
                return false;
            }
        } elseif ( false === strpos( $data, '"' ) ) {
            return false;
        }
        // Or else fall through.
    case 'a':
    case 'O':
        return (bool) preg_match( "/^{$token}:[0-9]+:/s", $data );
    case 'b':
    case 'i':
    case 'd':
        $end = $strict ? '$' : '';
        return (bool) preg_match( "/^{$token}:[0-9.E+-]+;$end/", $data );
}
return false;

}