simply adding code to php function

I want to create a php function with the following code but when I add this to a function it stops working:

Code works:

    $arr = array(
        "index.php" => $home, 
        "about.php" => $about, 
        "details.php" => $details
    );
    $url = basename($_SERVER['PHP_SELF']);

    foreach($arr as $key => $value){

        if($url == $key) {
            echo $value;
        }
    }
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Code Doesnt work:

function metaData() {
    $arr = array(
        "index.php" => $home, 
        "about.php" => $about, 
        "details.php" => $details
    );
    $url = basename($_SERVER['PHP_SELF']);

    foreach($arr as $key => $value){

        if($url == $key) {
            echo $value;
        }
    }
}

metaData(); // NULL
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