Here my little contribution for a simple yet handy debug function. Stolen from the php documentation. :)

php debug function #snippet

<?php 
/**
* dbug (mixed $expression [, mixed $expression [, $... ]])
* Author : dcz
* Feel free to use as you wish at your own risk ;-)
*/
function dbug() {
    static $output = '', $doc_root;
    $args = func_get_args();
    if (!empty($args) && $args[0] === 'print') {
        $_output = $output;
        $output = '';
        return $_output;
    }
    // do not repeat the obvious (matter of taste)
    if (!isset($doc_root)) {
        $doc_root = str_replace('\\', '/', $_SERVER['DOCUMENT_ROOT']);
    }
    $backtrace = debug_backtrace();
    // you may want not to htmlspecialchars here
    $line = htmlspecialchars($backtrace[0]['line']);
    $file = htmlspecialchars(str_replace(array('\\', $doc_root), array('/', ''), $backtrace[0]['file']));
    $class = !empty($backtrace[1]['class']) ? htmlspecialchars($backtrace[1]['class']) . '::' : '';
    $function = !empty($backtrace[1]['function']) ? htmlspecialchars($backtrace[1]['function']) . '() ' : '';
    $output .= "<b>$class$function =&gt;$file #$line</b><pre>";
    ob_start();
    foreach ($args as $arg) {
        var_dump($arg);
    }
    $output .= htmlspecialchars(ob_get_contents(), ENT_COMPAT, 'UTF-8');
    ob_end_clean();
    $output .= '</pre>';
}
?>

usage :
<?php 
dbug($scalar, $array, $object, $resource, CONSTANT);
//.. 
dbug($other);
//..
echo dbug('print'); // actually output the result of all previous calls
// looks like :
// class::method() =>/path/from/doc/root/file.php #line
// var_dump result

?>