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If I look through the libgd source (2.0.34RC1 or head)  "gh.h" I can find:
But these functions are completely missing in the bundled libgd 2.0.35 from PHP and thus are not avaliable.
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These functions should be available once Pierre merges the latest libgd sources into PHP, however their PHP binding could need some help implementing unless Pierre got something laying around from when it was implemented.
Duplicate of request #32803.