Description:
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In the sample code on imagecolorat() the function imagecolorsforindex() is missing an argument, the image resource. See the man page for imagecolorsforindex:

array imagecolorsforindex  ( resource $image  , int $index  )

Reproduce code:
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<?php
$im = imagecreatefrompng("php.png");
$rgb = imagecolorat($im, 10, 15);

list($r, $g, $b, $alpha) = imagecolorsforindex($rgb);

var_dump($r, $g, $b, $alpha);

Expected result:
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<?php
$im = imagecreatefrompng("php.png");
$rgb = imagecolorat($im, 10, 15);

list($r, $g, $b, $alpha) = imagecolorsforindex($im, $rgb);

var_dump($r, $g, $b, $alpha);