Contact Form 7: custom validation [closed]

Hope I did get it right:

<?php

function specialValidation($value) {
    $explode = explode(' ', $value);
    if(sizeof($explode) > 1) {
        echo 'ERROR: You have input more than one word.';
        return;
    }
    $mustBeS = substr($explode[0],0,1);
    if($mustBeS != 's' && $mustBeS != 'S') {
        echo 'ERROR: Your word must start with an "s" or "S".';
        return;
    }
}