Creating a Dynamic Vertical Gradient in PHP

create-image.php

<?php

// Tell the browser it's a GIF image.
header('Content-Type: image/gif');

/*
Create an image with a width of 1 and height of $height.
Width only needs to be 1px because it will presumably
be tiled horizontally.
*/
$gradient = imagecreatetruecolor(1, $height);

// For each pixel in the image...
for ($x = 0; $x < $height; $x++) {

  // Take the start color and increment it a step for every pixel.
  $red = $start[0] + $step[0] * $x;
  $green = $start[1] + $step[1] * $x;
  $blue = $start[2] + $step[2] * $x;

  // Create an image-readable color value.
  $color = imagecolorallocate($gradient, $red, $green, $blue);

  // Set the color of the pixel 0 from the left and
  // $x from the top to $color.
  imagesetpixel($gradient, 0, $x, $color);
}

// Output the image.
imagegif($gradient);

// Destroy the image from memory.
imagedestroy($gradient);

?>