Description:
------------
It would be nice to have an exif_imagetypefromstring function to determine an 
image type from a string without having to save the string to a file in order to 
use the exif_imagetype function.

There is a getimagesizefromstring function in PHP 5.4 but exif_imagetype is 
supposed to be much faster as it only needs to look at the first couple of bytes 
of the string to determine the image type. 

My problem is I can't really take advantage of the faster behavior because my 
image data is in a string, not in a file.

Test script:
---------------
<?php
$extension = image_type_to_extension(exif_imagetypefromstring($bin_data));

// versus

$tmpfile = tempnam('/tmp', 'upload');
file_put_contents($tmpfile, $bin_data);
$extension = image_type_to_extension(exif_imagetype($tmpfile));
unlink($tmpfile);