问题
I had this problem on a client server
$tmp = imagerotate($tmp, $angle, -1);
$tmp => bool(false) with no error message displayed
$tmp = imagerotate($tmp, $angle, imagecolorallocatealpha($tmp, 0, 0, 0 , 127));
this works fine
$tmp => resource(89) of type (gd)
What server configuration variable should be changed to make -1 work ?
GD Version: bundled (2.1.0 compatible)
libPNG Version: 1.2.44
回答1:
"What server configuration variable should be changed to make -1 work?"
There is no server configuration variable that makes this work. I ran in to the same issue as you (while using a third party library) and discovered that those who were passing in -1 were pretty much lucky that it did work. Based on my research I believe what happened was one person used this, it worked, and the code just got copied into various projects without a real understanding of it's function. I say this because I see the same exact code, including comments, in various projects.
Later versions of the gd library changed the behavior some, breaking the imagerotate function for those who were passing in -1 as the background color.
Anyway, the solution is to use imagecolorallocatealpha to add a transparent "color" to the image palette, and pass that color as the background color to imagerotate. Your solution of: $tmp = imagerotate($tmp, $angle, imagecolorallocatealpha($tmp, 0, 0, 0 , 127)); should be just fine.
来源：https://stackoverflow.com/questions/24643783/php-gd-imagerotate-fails-when-1-is-passed-for-bgd-color