Question

I’m trying to replace all spaces with underscores and the following is not working:

$id = "aa aa";
echo $id;
preg_replace('/\s+/', '_', $id);
echo $id;

prints

aa aaaa aa

Answer 1

The function preg_replace doesn’t modify the string in-place. It returns a new string with the result of the replacement. You should assign the result of the call back to the $id variable:

$id = preg_replace('/\s+/', '_', $id);