Load CSS files using PHP, based on page content

I would say you're over thinking this. Set a far away cache expiration date on your DataTable css and simply let the user cache the css file.

This could be done using output buffering, but it sounds like a bad idea to collect the whole document, analyze it, and then add a style sheet header - it's likely to be slow, kludgy and may even hit memory limits in the case of huge tables.

I would tend to say always load the CSS files, and see to it that they're properly cached.

I found it easiest to just set some flags in my parent .php file and have the header file check for those flags and modify output as it gets loaded.

index.php:

<?php
    $INCLUDE_TABLE_CSS = true;
    include('header.php');

header.php:

blah blah blah 
<?php if (isset($INCLUDE_TABLE_CSS) && $INCLUDE_TABLE_CSS) { ?>
   <link rel="stylesheet" .... href="table.css" />
<? } ?>
blah blah blah 

Unless you've got a large number of conditional settings, then this is fairly simple to manage.

Javascript is a better way to do this. You can do a deferred inclusion, sometimes called "lazy load". Within domready, you would check for the presence of a given class, let's say dataTable. If there are any elements with this class, you inject a new <script> or <link> tag into the header with a reference to the javascript or css file containing the needed script/styles. The <script>/<link> tag's onload event will be the callback to trigger whatever initialization you have to do once the script is in place.

My apologies that I can't tell you the jQuery way (I am a Mootools guy), but in Mootools there is a class called Asset that manages the creation of the <script>/<link> tag and the resulting onComplete event. I'm certain there is a jQuery analog to this.

Use jQuery! This code looks to see if there is a <table>...</table> object, and if there is, it creates a new <link> element with your desired CSS file and adds it to the header object.

if ($('table').length>0){
    var link = $("<link>");
    link.attr({type: 'text/css', rel: 'stylesheet', href: 'tableStyleSheet.css'});
    $("head").append( link ); 
}