在非常高的图像上使用PHP绘制线条，脚本停止绘制。怎么了，怎么解决？

I have a PHP script that creates a very tall image and draws a lot of lines on it (an organizational web kind of look). For the tallest images I've tried creating, the line drawing just stops abruptly toward the middle to bottom of the image: I have a PHP script that creates a very tall im