[Rasy]

Sonya,
такое?

$s = 'watch?v=aryXFA4MzSs&v1=asdaabdba';
$ar = ['watch?v=aasdbasd&v1=asdazzz', 'watch?v=aryXFA4MzSs&v1=asdaabdba'];    
$indx = key(preg_grep('|'.preg_quote($s).'|i',$ar));
echo $indx; // 1