<?php
	function getLuminance($val) {
		$hex = $val;
		list($r, $g, $b) = sscanf($hex, "#%02x%02x%02x");
		$red = 0.2126 * ($r/255);
		$green = 0.7152 * ($g/255);
		$blue = 0.0772 * ($b/255);

		if ( $red + $green + $blue <= .5 ){
			return 'light-text';
		} else {
			return 'dark-text';
		}
	}
?>

//Usage:
<div class="<?php echo getLuminance('#ffffff'); ?>"></div>
<div class="<?php echo getLuminance('#000000'); ?>"></div>

//Results in:
<div class="dark-text"></div>
<div class="light-text"></div>