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.

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