<?php
$s1 = "shrr.tk/1";

$kek = substr($s1, strpos($s1, "/") + 1);
if(is_numeric($kek)){
    echo $kek;
}

if(checkstart("shrr.tk/", $s1) === true){
    echo "true m8";
}

function checkstart($startstring, $string){
    if(substr($string, 0) === "$startstring"){
        return true;
    }
 return null;
}