What is this mark for “? function()” [closed]

That’s called Ternary Operator.

if( ! empty( $_POST['variable_b'] ) {
    $variable = explode( "\n", trim( $_POST['variable_b'] );
} else {
    $variable = array();
}

You can see PHP Shorthand If/Else Using Ternary Operators (?:) for more details.