<?php

echo $img_name2 = "my_beautiful_picture_123456_789101112131415_16171819202122.jpg";
echo " | ";
echo $img_name2 = substr($img_name2, 0, strpos($img_name2, "_"));
echo " | ";
echo $img_name2 = preg_replace('/_[0-9].*/', '', $img_name2);