Return the profile image (avatar) of a Twitter user using their RSS feed (no API call necessary).

get_twitter_avatar.php

<?php
function getTwitterAvatar( $username, $size = "normal" ) {
	// mini = 24px, normal = 48px, bigger = 73px
	$sizes = array( "mini", "normal", "bigger" );
	
	// Check for size, default to normal
	if( in_array( $size, $sizes ) ) {
		$variant = $size;
	} else {
		$variant = "normal";
	}
	
	// Get the user's profile image from their RSS feed
	$rawxml = simplexml_load_file( "http://twitter.com/users/{$username}.xml" );
	$imageurl = $rawxml->profile_image_url;
	
	// Return the image's url with the size requested
	if( $variant == "normal" ) {
		return $imageurl;
	} else {
		return str_replace( "normal", $variant, $imageurl );
	}
}
?>

I should also note that the php.ini on your server must have allow_url_fopen = on in order for this to work, otherwise PHP will throw an error on the simplexml_load_file() function.