Replacing Spaces with Underscores

Question

I have a PHP Script that users will enter a name like: Alex_Newton,

However, some users will use a space rather than an underscore, so my question is:

Answer 1

This is part of my code which makes spaces into underscores for naming my files:

$file = basename($_FILES['upload']['name']);
$file = str_replace(' ','_',$file);