imagecolortransparent

Sets the transparent color in the given image.

An image resource, returned by one of the image creation functions, such as imagecreatetruecolor().

A color identifier created with imagecolorallocate().

The identifier of the new (or current, if none is specified) transparent color is returned. If color is not specified, and the image has no transparent color, the returned identifier will be -1.

Transparency is copied only with imagecopymerge() and true color images, not with imagecopy() or pallete images.

The transparent color is a property of the image, transparency is not a property of the color. Once you have set a color to be the transparent color, any regions of the image in that color that were drawn previously will be transparent.

<?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);
?>