How to use IF Statement in WordPress?

Here you go..

add_action( 'gform_after_update_entry_7', 'add_length_on_update', 10, 2 );
function add_length_on_update( $form, $entry_id ) {
    if( !empty($_POST["input_3"]) && !empty($_POST["input_27"]) && !empty($_POST["input_28"]) ){
        // get input from form
        $date = $_POST["input_3"];
        $start = $_POST["input_27"];
        $end = $_POST["input_28"];

        // convert date and time arrays into datetime formats
        $startdate = DateTime::createFromFormat('m/d/Y@h:i a', $date . "@". $start[0].":".$start[1]." ".$start[2]);
        $enddate =   DateTime::createFromFormat('m/d/Y@h:i a', $date . "@". $end[0].":".$end[1]." ".$end[2]);

        //convert datetimes into seconds to compare
        $starttime = strtotime($startdate->format('Y-m-d H:i:s'));
        $endtime = strtotime($enddate->format('Y-m-d H:i:s'));

        // check to see if the times span overnight
        if($starttime > $endtime) {
            $endtime = strtotime($enddate->format('Y-m-d H:i:s') . " +1 day");
        }

        // perform calculation
        $diff = floor(($endtime - $starttime)/30);

        GFAPI::update_entry_field( $entry_id, 32, $diff );
    }   
}