PHP / vBulletin - Passing a variable through a function and replacing a variable with the output

I am trying to pass the forum title variable through a function then replace the variable for the title with the output. I have read the documentation and have successfully echoed on the page, but when I try to implement my variables is where I am strugling.

I have created a plugin at global_start which contains the function:

function seoUrl($string) {
    //Lower case everything
    $string = strtolower($string);
    //Make alphanumeric (removes all other characters)
    $string = preg_replace("/[^a-z0-9_\s-]/", "", $string);
    //Clean up multiple dashes or whitespaces
    $string = preg_replace("/[\s-]+/", " ", $string);
    //Convert whitespaces and underscore to dash
    $string = preg_replace("/[\s_]/", "-", $string);
}

I then created another plugin at forumbit_display:

$san = 'seoUrl';
$san($forum[title]);
$forumname = $san($forum[title]);

Yet my variable $forumname is not putting out any output.

I have achieved a slim down version of this successfully:

$forumname = strtolower(str_replace(' ', '-', $forum[title]));

I just would like some help getting over this learning curve with going through functions. How come I am not seeing any output?

Solution

You need to return the string after modification.

function seoUrl($string) {
    //Lower case everything
    $string = strtolower($string);
    //Make alphanumeric (removes all other characters)
    $string = preg_replace("/[^a-z0-9_\s-]/", "", $string);
    //Clean up multiple dashes or whitespaces
    $string = preg_replace("/[\s-]+/", " ", $string);
    //Convert whitespaces and underscore to dash
    $string = preg_replace("/[\s_]/", "-", $string);
   return $string; 
}

Note When you pass a string as function parameter, a copy of the value is passed to your function. Therefore any modifications made in the function does not affect the original variable outside the function unless you pass it by reference.

How to get the reference: prefix the argument with &

function seoUrl(&$string) {
    //Lower case everything
    $string = strtolower($string);
    //Make alphanumeric (removes all other characters)
    $string = preg_replace("/[^a-z0-9_\s-]/", "", $string);
    //Clean up multiple dashes or whitespaces
    $string = preg_replace("/[\s-]+/", " ", $string);
    //Convert whitespaces and underscore to dash
    $string = preg_replace("/[\s_]/", "-", $string);
}

Now the above will work without returning the modified string.