- 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";