php reduction png image without loss of transparent color solution

There are mainly two ways to use the gd library:

imagecolorallocatealpha // Distribution of color  + alpha
imagesavealpha // Set in save  png  When the image is saved intact  alpha  The channel information 

Code examples:

// Access to the source diagram gd Image identifier 
$srcImg = imagecreatefrompng('./src.png');
$srcWidth = imagesx($srcImg);
$srcHeight = imagesy($srcImg);
// Create a new 
$newWidth = round($srcWidth / 2);
$newHeight = round($srcHeight / 2);
$newImg = imagecreatetruecolor($newWidth, $newHeight);
// Distribution of color  + alpha , fill the new drawing with color 
$alpha = imagecolorallocatealpha($newImg, 0, 0, 0, 127);
imagefill($newImg, 0, 0, $alpha);
// Copy the source diagram to the new diagram and set it to save  PNG  When the image is saved intact  alpha  The channel information 
imagecopyresampled($newImg, $srcImg, 0, 0, 0, 0, $newWidth, $newHeight, $srcWidth, $srcHeight);
imagesavealpha($newImg, true);
imagepng($newImg, './dst.png');