$re = '/(?<=click=)([^&]+)/m';
$str = 'https://play.google.com/store/apps/details?id=air.com.littlebigplay.games.premium.plurals&click=541255b82bc957475c7b6222d1a980a5&mysuffix={HELLO_WORLD}&optid=541255b82bc957475c7b';
preg_match_all($re, $str, $matches, PREG_SET_ORDER, 0);
// Print the entire match result
var_dump($matches);
Please keep in mind that these code samples are automatically generated and are not guaranteed to work. If you find any syntax errors, feel free to submit a bug report. For a full regex reference for PHP, please visit: http://php.net/manual/en/ref.pcre.php