Only variables should be passed by reference

Everyone else has already given you the reason you're getting an error, but here's the best way to do what you want to do: $file_extension = pathinfo($file_name, PATHINFO_EXTENSION);

Assign the result of explode to a variable and pass that variable to end:

$tmp = explode('.', $file_name);
$file_extension = end($tmp);

The problem is, that end requires a reference, because it modifies the internal representation of the array (i.e. it makes the current element pointer point to the last element).

The result of explode('.', $file_name) cannot be turned into a reference. This is a restriction in the PHP language, that probably exists for simplicity reasons.