imagecropauto

In some cases the use of the IMG_CROP_WHITE or IMG_CROP_BLACK does not work. The function returns FALSE. It is best to use the IMG_CROP_THRESHOLD mode and specify the color in fourth argument as in the example below :
<?php
$original_img = imagecreatefromjpeg($image_path);
// Use this :
$cropped_img_white = imagecropauto($original_img , IMG_CROP_THRESHOLD, null, 16777215);
// Rather than :
$cropped_img_white = imagecropauto($original_img , IMG_CROP_WHITE);
// AND
// Use this :
$cropped_img_black = imagecropauto($original_img , IMG_CROP_THRESHOLD, null, 0);
// Rather than :
$cropped_img_black = imagecropauto($original_img , IMG_CROP_BLACK);
?>

I don’t know why you can’t set the threshold for the four sides filter (IMG_CROP_SIDES) so here’s how to do it manually using the IMG_CROP_THRESHOLD filter instead.
$threshold = .5;
$im = imagecreatefromjpeg('somefile.jpg');
$width = imagesx($im);
$height = imagesy($im);
$arr = [
 [0,0],
 [$width-1,0],
 [0,$height-1],
 [$width-1,$height-1],
];
$red = 0;
$green = 0;
$blue = 0;
// grab the colours from all four corners
foreach( $arr as $arr2 ) {
 $thisColor = imagecolorat($im, $arr2[0], $arr2[1]); 
 $rgb = imagecolorsforindex($im, $thisColor); 
 $red += round(round(($rgb['red'] / 0x33)) * 0x33); 
 $green += round(round(($rgb['green'] / 0x33)) * 0x33); 
 $blue += round(round(($rgb['blue'] / 0x33)) * 0x33); 
}
// and average them
$red /= 4;
$green /= 4;
$blue /= 4;
$newColor = imagecolorallocate($im, $red, $green, $blue); 
$cropped = imagecropauto($im, IMG_CROP_THRESHOLD, $threshold, $newColor);
imagejpg($cropped, 'somefile.cropped.jpg');
imagedestroy($im);
imagedestroy($cropped);