See http://stackoverflow.com/a/10524505/624466

ParseYoutubeUrlTest.php

<?php

// Function `parse_yturl()` from <http://stackoverflow.com/a/10524505/624466>

/**
 *  Check if the input string is a valid YouTube URL
 *  and try to extract the YouTube Video ID from it.
 * 
 *  @author  Stephan Schmitz <eyecatchup@gmail.com>
 *  @param   $url   string   The string that shall be checked.
 *  @return  mixed           YouTube Video ID, or (boolean) false.
 */
function parse_yturl($url) {
    $pattern = '#^(?:https?://|//)?' # Optional URL scheme. Either http, or https, or protocol-relative.
             . '(?:www\.|m\.)?'      #  Optional www or m subdomain.
             . '(?:'                 #  Group host alternatives:
             .   'youtu\.be/'        #    Either youtu.be,
             .   '|youtube\.com/'    #    or youtube.com
             .     '(?:'             #    Group path alternatives:
             .       'embed/'        #      Either /embed/,
             .       '|v/'           #      or /v/,
             .       '|watch\?v='    #      or /watch?v=,
             .       '|watch\?.+&v=' #      or /watch?other_param&v=
             .     ')'               #    End path alternatives.
             . ')'                   #  End host alternatives.
             . '([\w-]{11})'         # 11 characters (Length of Youtube video ids).
             . '(?![\w-])#';         # Rejects if overlong id.
    preg_match($pattern, $url, $matches);
    return (isset($matches[1])) ? $matches[1] : false;
}

// Tests for function `parse_yturl()`

$testIds = [
    'l8qdouU9cYE',
    '_PqP1qHhsyM',
    '3G3_im4RyI8',
    'uqU4Mkd-_gU',
];

$testUrls = [
    // Without scheme and subdomain (Domain: youtu.be, Path: /)
    'youtu.be/%s',
    // Without scheme, with subdomain (Domain: youtu.be, Path: /)
    'www.youtu.be/%s',
    // With HTTP scheme, without subdomain (Domain: youtu.be, Path: /)
    'http://youtu.be/%s',
    // With HTTP scheme and subdomain (Domain: youtu.be, Path: /)
    'http://www.youtu.be/%s',
    // With HTTPS scheme, without subdomain (Domain: youtu.be, Path: /)
    'https://youtu.be/%s',
    // With HTTPS scheme and subdomain (Domain: youtu.be, Path: /)
    'https://www.youtu.be/%s',
    // Without scheme and subdomain (Domain: youtube.com, Path: /embed)
    'youtube.com/embed/%s',
    'youtube.com/embed/%s&other_params',
    // Without scheme, with subdomain (Domain: youtube.com, Path: /embed)
    'www.youtube.com/embed/%s',
    'www.youtube.com/embed/%s&other_params',
    // With HTTP scheme, without subdomain (Domain: youtube.com, Path: /embed)
    'http://youtube.com/embed/%s',
    'http://youtube.com/embed/%s&other_params',
    // With HTTP scheme and subdomain (Domain: youtube.com, Path: /embed)
    'http://www.youtube.com/embed/%s',
    'http://www.youtube.com/embed/%s&other_params',
    // With HTTPS scheme, without subdomain (Domain: youtube.com, Path: /embed)
    'https://youtube.com/embed/%s',
    'https://youtube.com/embed/%s&other_params',
    // With HTTPS scheme and subdomain (Domain: youtube.com, Path: /embed)
    'https://www.youtube.com/embed/%s',
    'https://www.youtube.com/embed/%s&other_params',
    // Without scheme and subdomain (Domain: youtube.com, Path: /v)
    'youtube.com/v/%s',
    'youtube.com/v/%s&other_params',
    // Without scheme, with subdomain (Domain: youtube.com, Path: /v)
    'www.youtube.com/v/%s',
    'www.youtube.com/v/%s&other_params',
    // With HTTP scheme, without subdomain (Domain: youtube.com, Path: /v)
    'http://youtube.com/v/%s',
    'http://youtube.com/v/%s&other_params',
    // With HTTP scheme and subdomain (Domain: youtube.com, Path: /v)
    'http://www.youtube.com/v/%s',
    'http://www.youtube.com/v/%s&other_params',
    // With HTTPS scheme, without subdomain (Domain: youtube.com, Path: /v)
    'https://youtube.com/v/%s',
    'https://youtube.com/v/%s&other_params',
    // With HTTPS scheme and subdomain (Domain: youtube.com, Path: /v)
    'https://www.youtube.com/v/%s',
    'https://www.youtube.com/v/%s&other_params',
    // Without scheme and subdomain (Domain: youtube.com, Path: /watch)
    'youtube.com/watch?v=%s',
    'youtube.com/watch?v=%s&other_params',
    'youtube.com/watch?other_params&v=%s',
    'youtube.com/watch?other_params&v=%s&more_params',
    // Without scheme, with subdomain (Domain: youtube.com, Path: /watch)
    'www.youtube.com/watch?v=%s',
    'www.youtube.com/watch?v=%s&other_params',
    'www.youtube.com/watch?other_params&v=%s',
    'www.youtube.com/watch?other_params&v=%s&more_params',
    // With HTTP scheme, without subdomain (Domain: youtube.com, Path: /watch)
    'http://youtube.com/watch?v=%s',
    'http://youtube.com/watch?v=%s&other_params',
    'http://youtube.com/watch?other_params&v=%s',
    'http://youtube.com/watch?other_params&v=%s&more_params',
    // With HTTP scheme and subdomain (Domain: youtube.com, Path: /watch)
    'http://www.youtube.com/watch?v=%s',
    'http://www.youtube.com/watch?v=%s&other_params',
    'http://www.youtube.com/watch?other_params&v=%s',
    'http://www.youtube.com/watch?other_params&v=%s&more_params',
    // With HTTPS scheme, without subdomain (Domain: youtube.com, Path: /watch)
    'https://youtube.com/watch?v=%s',
    'https://youtube.com/watch?v=%s&other_params',
    'https://youtube.com/watch?other_params&v=%s',
    'https://youtube.com/watch?other_params&v=%s&more_params',
    // With HTTPS scheme and subdomain (Domain: youtube.com, Path: /watch)
    'https://www.youtube.com/watch?v=%s',
    'https://www.youtube.com/watch?v=%s&other_params',
    'https://www.youtube.com/watch?other_params&v=%s',
    'https://www.youtube.com/watch?other_params&v=%s&more_params',
];

$results = [];

foreach ($testIds as $id) {
    $results[$id] = [];

    foreach ($testUrls as $str) {
        $testString = sprintf($str, $id);
        array_push($results[$id], [
            'str' => $testString,
            'id' => parse_yturl($testString),
        ]);
    }
}

$matches = 0;
$errors = 0;

//header('Content-type: text/plain;');
foreach ($results as $key => $val) {
    $testId = $key;
    printf('# Testing strings for id "%s"' . PHP_EOL . PHP_EOL, $testId);

    foreach ($val as $res) {
        if ($res['id'] !== $testId) {
            $errors++;
            $result = 'no match';
        } else {
            $matches++;
            $result = $res['id'];
        }
        printf('  Result: %s, String: %s' . PHP_EOL, $result, $res['str']);
    }
    print PHP_EOL;
}

printf('Done. %d assertation(s), %d error(s)' . PHP_EOL, $matches, $errors);