Do you spot the bug?

preg_bug.php

<?php 

$string = 'lalalalala';
$string = preg_replace('/(a+)/', 'many-a', $string);

// continue my work with string
// hopefully it is "lmany-almany-almany-almany-almany-a"

?>

preg_right.php

<?php 
 
$string = 'lalalalala';
$preg_ret = preg_replace('/(a+)/', 'many-a', $string);
 
if ($preg_ret === null) {
    throw new Exception("Preg replace did an error: ", preg_last_error());
}

$string = $preg_ret;

?>