Using imagefill()
<?php
header("Content-type: image/gif");
$image = imagecreate( 200, 200 );
$red = imagecolorallocate($image, 255,0,0);
$blue = imagecolorallocate($image, 0,0,255 );
imageline( $image, 0, 0, 199, 199, $blue );
imagefill( $image, 0, 199, $blue );
imagegif($image);
?>
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