PHP- Creating and image and reading it gives different values - php

Question

While working on the images in PHP, i noticed a strange behavior, not sure why.
I read a jpeg image, then create a new image from it. Then I read the new image. But, displaying the color values give different results. (Though, images look same.)
Here is my code:
$img = imagecreatefromjpeg('test2.jpg');
imagejpeg($img, 'test100.jpg');
$new= imagecreatefromjpeg('test100.jpg');
list($width,$height) = getimagesize('test100.jpg');
for($x=0; $x<$width; $x++){
for($y=0; $y<$height; $y++){
echo "At [$x][$y] --".imagecolorat($img,$x,$y)."--".imagecolorat($new,$x,$y)."<br>";
}
}
I get the following output:
At [0][0] --14732734--14667454
At [0][1] --14601148--14666941
At [0][2] --13810862--13810862
At [0][3] --14271156--14139570
At [0][4] --15717832--15652039
At [0][5] --14598838--14401972
At [0][6] --11440262--11309188
And so on...
I'm not getting the why the values are getting changed, though images look similar.
Please help.
Thank you

Answer 1

JPEG is an image file format with lossy compression. That means you will loose data each time you save an image.
That you see different color values per pixel means nothing different than data-loss. Instead of opening and re-saving, just copy the jpeg file with the file-system functions
$img = imagecreatefromjpeg('test2.jpg');
copy('test2.jpg', 'test100.jpg'); ## <-- copy image _file_
$new= imagecreatefromjpeg('test100.jpg');
you will then get the identical results:
At [0][0] --855561--855561
At [0][1] --855561--855561
At [0][2] --921353--921353
At [0][3] --987144--987144
At [0][4] --1052937--1052937
At [0][5] --1250570--1250570
At [0][6] --1579535--1579535

Answer 2

The quality of the image saved to test100.jpg would be default at 75% if no value is specified when saving... use the following to save 100% quality.
imagejpeg($img, 'test100.jpg', 100);