<?php
function strengthPlan( $data ) {

    $data = ( int ) $data;
    if($data <= 10) return 'first';
    if($data <= 20) return 'second'; // will only get so far if $data > 10, because in case $data <= 10 something has been returned already
    // etc.
}

echo strengthPlan( 0 );
?>

first