- strpos: documentation ( source)
<?php
echo "str starts with 0" . (strpos('CNXXXXXX122334.txt','CN') == 0) . "\n";
echo "str does not starts with 0" . (strpos('CNXXXXXX122334.txt','XT') == 0) . "\n";
echo "str starts with 0" . (strpos('CNXXXXXX122334.txt','XT') == 1) . "\n";