From http://phpsnips.com/486/PHP-Tag-Cloud#.VBAXI_mSySo

php_tag_cloud.php

<?php function getCloud( $data = array(), $minFontSize = 12, $maxFontSize = 30 )   
{   
    $minimumCount = min($data);   
    $maximumCount = max($data);   
    $spread       = $maximumCount - $minimumCount;   
    $cloudHTML    = '';   
    $cloudTags    = array();   
   
    $spread == 0 && $spread = 1;   
   
    foreach( $data as $tag => $count )   
    {   
        $size = $minFontSize + ( $count - $minimumCount )   
            * ( $maxFontSize - $minFontSize ) / $spread;   
        $cloudTags[] = '<a style="font-size: ' . floor( $size ) . 'px'   
        . '" class="tag_cloud" href="#" title="\'' . $tag  .   
        '\' returned a count of ' . $count . '">'   
        . htmlspecialchars( stripslashes( $tag ) ) . '</a>';   
    }   
   
    return join( "\n", $cloudTags ) . "\n";   
}

$arr = Array('Actionscript' => 35, 'Adobe' => 22, 'Array' => 44, 'Background' => 43,   
    'Blur' => 18, 'Canvas' => 33, 'Class' => 15, 'Color Palette' => 11, 'Crop' => 42,   
    'Delimiter' => 13, 'Depth' => 34, 'Design' => 8, 'Encode' => 12, 'Encryption' => 30,   
    'Extract' => 28, 'Filters' => 42);   
echo getCloud($arr, 12, 36);