gistfile1.sh

$ gtime php substrstrlen.php
0.43 user 
0.01 system 
0:00.56 elapsed

0.43 user 
0.01 system 
0:00.50 elapsed

0.42 user 
0.01 system 
0:00.47 elapsed 

$ gtime php strpos.php
0.27 user 
0.01 system 
0:00.30 elapsed 

0.27 user 
0.00 system 
0:00.29 elapsed

0.28 user 
0.01 system 
0:00.30 elapsed 

strpos.php

<?php 

$class = 'Symfony\Bundle\FrameworkBundle\DependencyInjection\Configuration';
$prefix = 'Twig_';

for ($i = 0; $i < 1000000; ++$i) {
    if (0 === strpos($class, $prefix)) {}
}

substrstrlen.php

<?php

$class = 'Symfony\Bundle\FrameworkBundle\DependencyInjection\Configuration';
$prefix = 'Twig_';

for ($i = 0; $i < 1000000; ++$i) {
    if (substr($class, 0, strlen($prefix)) === $prefix) {}
}

you should move the strlen() out of the loop...