I personally like PHP the most, but javascript is also a great tool.
PHP may not be the best and most efficient language, but it sure is the easiest to learn and can get the job done.
Here is a code I recently put together for a client that increased their profits:
<?php
if ( wp_is_mobile() ) {
if(date("Hi") < 1800 && date("Hi") > 800){
echo "<center><h2>Call <b>(555) 555-5555</b> for Free Consultation</h2></center>";
}
else {
echo $consultationform;
}
}
?>
If the user is visiting the Wordpress site on a Mobile phone and hours are between 8AM and 6PM then display Phone Number, otherwise show Consultation Form.
This increased calls because Mobile users were more likely to click the phone number link than fill out the form.