首先让我们来看下官方文档的说明：
imagecreate — Create a new palette based image
resource imagecreate ( int$width, int$height)imagecreate() returns an image identifier representing a blank image of specified size.
We recommend the use of imagecreatetruecolor().
imagecreatetruecolor — Create a new true color image
resource imagecreatetruecolor ( int
$width, int$height)imagecreatetruecolor() returns an image identifier representing a black image of the specified size.
imagecolorallocate — Allocate a color for an image
int imagecolorallocate ( resource$image, int$red, int$green, int$blue)Returns a color identifier representing the color composed of the given RGB components.
imagecolorallocate() must be called to create each color that is to be used in the image represented by
image.Note:
The first call to imagecolorallocate() fills the background color in palette-based images - images created using imagecreate().
意思是使用imagecreate()创建图像，第一次使用Imagecolorallcote时，所赋给的颜色会自动认为是背景色）
- 用imagecreatetruecolor()创建图像（jpeg,png等，不支持gif）时，图片默认背景颜色是黑色，要想改变背景色，必须使用别的指令如 imagefill() 填充背景。步骤如下：
- 1.创建真彩图片
- 2.分配颜色
- 3.填充颜色。
<?php
header("Content-type:image/png");
$image=imagecreatetruecolor(120,20) or die(" can not");
$background_color=imagecolorallocate($image,0,255,0);
imagefill($image,0,0,$background_color);
$text_color=imagecolorallocate($image,255,0,0);
imagestring($image,1,5,5,"a simple text string",$text_color);
imagepng($image);
imagedestroy($image);
?>
- 用Imagecreate（）创建图像时，默认是空白，只要第一次分配颜色，所分配的颜色就是背景色了。
<?php
header ('Content-Type: image/png');
$im = imagecreate(120, 20)
or die('Cannot Initialize new GD image stream');
$background=imagecolorallocate($im,255,0,255);
$text_color = imagecolorallocate($im, 255, 0, 0);
imagestring($im, 1, 5, 5, 'A Simple Text String', $text_color);
imagepng($im);
imagedestroy($im);
?>