For those looking to grab the resolution of a JPEG image without using GD nor ImageMagic... I wrote this simple function.
Too bad GD doesn't have this very simple function for us to use...
<?php
function getJPEGresolution($filename){
$outRez=array();
// Read the file
ob_start(); // start a new output buffer
$image = file_get_contents($filename);
// grab DPI information from the JPG header
$outRez["xDPI"] = ord($image[15]);
$outRez["yDPI"] = ord($image[17]);
ob_end_clean(); // stop this output buffer
//xDPI and yDPI should equal in value... but we output both anyway...
return($outRez);
}//end function getJPEGresolution
?>