Using the 24-bit color Alpha Channel
In an RGB palette, the alpha channel is used to specify the "transparency" of a given color in the palette when it is rendered. Note: Alpha channels only apply to 24-bit canvases

imagecolorallocatealpha($img_r, $red, $green, $blue, $alpha);

Where $alpha is a value anywhere from 0 (see-through) to 127 (Solid)

Alpha chaneling handling
<?php

    define("WIDTH", 450);
    define("HEIGHT", 450);

    $img = imagecreatetruecolor(WIDTH, HEIGHT);

    $white = imagecolorallocate($img, 0xFF, 0xFF, 0xFF);
    $yellow = imagecolorallocate($img, 0xFF, 0xFF, 00);
    $red = imagecolorallocate($img, 0xFF, 0, 0);
    $blue_t   = imagecolorallocatealpha($img, 0, 0, 0xFF, 0x40);

    imagefill($img, 1, 1, $white);

    imageline($img, 0,0, WIDTH-1, HEIGHT-1, $blue_t);

    imagefilledrectangle($img, (WIDTH/2)-50, (HEIGHT/2)-50,
                               (WIDTH/2)+50, (HEIGHT/2)+50, $yellow);
    imagefilledrectangle($img, (WIDTH/2)-30, (HEIGHT/2)-30,
                               (WIDTH/2)+30, (HEIGHT/2)+30, $red);
    imagefilledrectangle($img, 10, 10, WIDTH-11, HEIGHT-11, $blue_t);
     
    header("Content-Type: image/png");
    imagepng($img);
?>