Functions Filter Question [closed]

this is for Variable scope.

A variable declared within a function has a LOCAL SCOPE and can only be accessed within that function

A variable declared outside a function has a GLOBAL SCOPE and can only be accessed outside a function:

$meinungen = Test; // is in global scope
function wpseo_set_title_callback($input) {
if (is_single()) {
    return ''. $meinungen . ''. $input . ' '. $input . ''; // now $meinungen local variable which you don not assign value.
}
// return default
return $input; }

to call the value just put global with in function

function wpseo_set_title_callback($input) {
if (is_single()) {
   global $meinungen;
    return ''. $meinungen . ''. $input . ' '. $input . '';
}
// return default
return $input;

}