variable not returning in function

[harmor19]

I'm sort of copying vBulletin's code that cleans data. If I echo the variable in the function it works but if I return the variable and echo it outside the function it doesn't.

Here is the code.

PHP Code:

<?php


function clean($mthd, $arr)
{
    
    if($mthd == "p")
    {
        
        foreach (array_keys($arr) as $val) 
        {
               go_clean($val, 'p');
        }
    }
}

function go_clean(&$val, $mthd)
{
    
    $efinfo = array();
    
    if($mthd == "p")
    {
      $efinfo[$val] = $_POST[$val];
    }
      
     return $efinfo; 
     
     //echo "$val: ". $_POST[$val] ."<br /> ";
    
}

$_POST['username'] = "harmor";
$_POST['password'] = "andrew123";

$data = array('username' => 'STR', 'password' => 'STR', 'age' => 'INT', 'email' => 'STR');
clean('p', $data);

echo $efinfo['username'];


?>

What does the "and" symbol in this function do? go_clean(&$val, $mthd)
I just added it to see if it changed anything but it didn't.