I’m working on a project that sends transactional emails from templates using variables in the %%varname%% format, but defines variables in another table as a lookup value, or query. So, I needed to match strings of type %%varname%% which screams regular expressions, but also needed varname to look up what to replace it with. So, instead of using multiple functions, I was able to do this with a callback and preg_replace_callback.

<?php
$string = "A complicated gentleman allow me to present,
Of all %%asdf%% the arts and faculties the terse embodiment,
He's a great arithmetician who can demonstrate with ease %%name%%
That two and two are three, or five, or anything you please;
An eminent Logician who can make it clear to you
That black is white – when looked at from the proper point of view;
A marvelous Philologist who'll undertake to show %%asdf%%
That 'yes' is but another and a neater form of 'no'.";

echo preg_replace_callback('(%%.*%%)', 
	create_function(
	    '$matches',
	    'return getValue($matches)'
	),
	$string
);

function getValue($var) { 
	return toupper(str_replace('%%', '', $var));
}

/* Outputs:
A complicated gentleman allow me to present,
Of all ASDF the arts and faculties the terse embodiment,
He's a great arithmetician who can demonstrate with ease NAME
That two and two are three, or five, or anything you please;
An eminent Logician who can make it clear to you
That black is white – when looked at from the proper point of view;
A marvelous Philologist who'll undertake to show ASDF
That 'yes' is but another and a neater form of 'no'.
*/

I use the function getValue to query the database and get the value of the variable. This function is pretty slick; now, PHP just needs to support callbacks that aren’t created as strings and we’ll be all set.