$data = "123_String";
$whatIWant = substr($data, strpos($data, "_") + 1);
echo $whatIWant;
$string = "Lorem ipsum dolor sit amet, consectetur adipiscing elit. Nulla felis diam, mattis id elementum eget, ullamcorper et purus.";
$prefix = "Nulla";
$index = strpos($string, $prefix) + strlen($prefix);
$result = substr($string, $index);
<?php
$my_url = 'http://www.example.com/5478631';
echo substr($my_url, strrpos($my_url, '/' )+1)."
";
?>
// print: 5478631