PHP: imagepng() and transparency

Question

When using the function imagepng() in php, how can I make sure the images that I save are saved with a transparent background?

Answer 1

Simply do this:

imagealphablending($img, false);
imagesavealpha($img, true);

Before outputting. Make sure that all source files (if you used any) are set to PNG 32-bit with transparency - if not the output may differ with black background or transparency does not comply.

Answer 2

There's a function called imagecolortransparent that allows you to set which color is made transparent. I don't know if this answers your question.

Answer 3

Here is an example of the imagecolortransparent function (if it helps):

<?php
// Create a 55x30 image
$im = imagecreatetruecolor(55, 30);
$red = imagecolorallocate($im, 255, 0, 0);
$black = imagecolorallocate($im, 0, 0, 0);

// Make the background transparent
imagecolortransparent($im, $black);

// Draw a red rectangle
imagefilledrectangle($im, 4, 4, 50, 25, $red);

// Save the image
imagepng($im, './imagecolortransparent.png');
imagedestroy($im);
?>

from the PHP manual: http://php.net/manual/en/function.imagecolortransparent.php