I think what you are missing is that you have to put two of them. So if hello\world is in the variable $string:
$new_string = preg_replace("/\/", " ", $string);
Or, the easier way is with str_replace for this example:
$new_string = str_replace("\", " ", $string);
The backslash () is the escape character for representing non-alphanumeric characters (such as \n is a line break and \r is a line feed). But to get a literal backslash, you must escape the backslash, thus the double backslashes.
Hope that helps!
Chris King