Resize image propotionally in PHP - php

Question

Bear with me, im new to PHP. I got this very nice tutorial from here:
Add watermark image PHP - developphp.com
The problem is, the cropping ratio is not proportionate. If I have wider image, it crop 500px width and the height is not proportion to the width the same as the height (tall image crop to 500px and width is lesser px.)
I want to make it proportional as 500px both width and height no matter how wide or tall the image is and should crop center.
Been searching and trying other method but failed. Hope someone can help. Thanks!
Here is the code:
<?php
// Access the $_FILES global variable for this specific file being uploaded
// and create local PHP variables from the $_FILES array of information
$fileName = $_FILES["uploaded_file"]["name"]; // The file name
$fileTmpLoc = $_FILES["uploaded_file"]["tmp_name"]; // File in the PHP tmp folder
$fileType = $_FILES["uploaded_file"]["type"]; // The type of file it is
$fileSize = $_FILES["uploaded_file"]["size"]; // File size in bytes
$fileErrorMsg = $_FILES["uploaded_file"]["error"]; // 0 for false... and 1 for true
$fileName = preg_replace('#[^a-z.0-9]#i', '', $fileName); // filter
$kaboom = explode(".", $fileName); // Split file name into an array using the dot
$fileExt = end($kaboom); // Now target the last array element to get the file extension
// START PHP Image Upload Error Handling -------------------------------
if (!$fileTmpLoc) { // if file not chosen
echo "ERROR: Please browse for a file before clicking the upload button.";
exit();
} else if($fileSize > 5242880) { // if file size is larger than 5 Megabytes
echo "ERROR: Your file was larger than 5 Megabytes in size.";
unlink($fileTmpLoc); // Remove the uploaded file from the PHP temp folder
exit();
} else if (!preg_match("/.(gif|jpg|png)$/i", $fileName) ) {
// This condition is only if you wish to allow uploading of specific file types
echo "ERROR: Your image was not .gif, .jpg, or .png.";
unlink($fileTmpLoc); // Remove the uploaded file from the PHP temp folder
exit();
} else if ($fileErrorMsg == 1) { // if file upload error key is equal to 1
echo "ERROR: An error occured while processing the file. Try again.";
exit();
}
// END PHP Image Upload Error Handling ---------------------------------
// Place it into your "uploads" folder mow using the move_uploaded_file() function
$moveResult = move_uploaded_file($fileTmpLoc, "uploads/$fileName");
// Check to make sure the move result is true before continuing
if ($moveResult != true) {
echo "ERROR: File not uploaded. Try again.";
exit();
}
// Include the file that houses all of our custom image functions
include_once("ak_php_img_lib_1.0.php");
// ---------- Start Universal Image Resizing Function --------
$target_file = "uploads/$fileName";
$resized_file = "uploads/resized_$fileName";
$wmax = 500;
$hmax = 500;
ak_img_resize($target_file, $resized_file, $wmax, $hmax, $fileExt);
// ----------- End Universal Image Resizing Function ----------
// ---------- Start Convert to JPG Function --------
if (strtolower($fileExt) != "jpg") {
$target_file = "uploads/resized_$fileName";
$new_jpg = "uploads/resized_".$kaboom[0].".jpg";
ak_img_convert_to_jpg($target_file, $new_jpg, $fileExt);
}
// ----------- End Convert to JPG Function -----------
// ---------- Start Image Watermark Function --------
$target_file = "uploads/resized_".$kaboom[0].".jpg";
$wtrmrk_file = "watermark.png";
$new_file = "uploads/protected_".$kaboom[0].".jpg";
ak_img_watermark($target_file, $wtrmrk_file, $new_file);
// ----------- End Image Watermark Function -----------
// Display things to the page so you can see what is happening for testing purposes
echo "The file named <strong>$fileName</strong> uploaded successfuly.<br /><br />";
echo "It is <strong>$fileSize</strong> bytes in size.<br /><br />";
echo "It is an <strong>$fileType</strong> type of file.<br /><br />";
echo "The file extension is <strong>$fileExt</strong><br /><br />";
echo "The Error Message output for this upload is: $fileErrorMsg<br />";
?>