PHP Tutorial: The Imagecolorset function of the GD library to simply modify the picture color

Now there is a single photo with a background color of pure blue (or red, etc.) and in contrast to the characters in the photo, the blue background in the picture is treated as white in PHP. That is, similar to PS, the effect of filling blue with white.

Use the Imagecolorset function of the GD library to modify a simple index color. But it's only valid for GIF and PNG pictures.

<?php
$img = file_get_contents (' http://picture address);
$im = imagecreatefromstring ($img);
$BG = Imagecolorat ($im, 0, 0);
Imagecolorset ($im, $BG, 0, 0, 255);
Imagepng ($im);
Imagedestroy ($im);
?>