php不能生成图片的解决办法

//设置 验证码高度宽度\上面字符个数

$img_w = 70;

$img_h = 22;

$font = 5;

$char_len = 5;

//数组合并， range()函数返回一个范围数组

$char = array_merge ( range ( 'a', 'z' ), range ( 'A', 'Z' ), range ( '1', '9' ) );

$rand_keys = array_rand ( $char, $char_len ); //随机从数组中取指定个数的元素,生成键值

if ($char_len == 1) { //若只有一个数，则array_rand()返回非数组类型

         $rand_keys = array ($rand_keys );

}

shuffle($rand_keys);  //可以不用

$code = '';

foreach ( $rand_keys as $k ) {

         $code .= $char [$k];

}

session_start ();

$_SESSION ['captcha'] = $code;

//添加线、色

//创建新图像

$img = imagecreatetruecolor ( $img_w, $img_h );

//分配颜色

$bg_color = imagecolorallocate ( $img, 0xcc, 0xcc, 0xcc );

//画布背景色

imagefill ( $img, 0, 0, $bg_color );

//干扰线

for($i = 0; $i < 300; ++$i) {

         $color = imagecolorallocate ( $img, mt_rand ( 0, 255 ), mt_rand ( 0, 255 ), mt_rand ( 0, 255 ) );

         imagesetpixel ( $img, mt_rand ( 0, $img_w ), mt_rand ( 0, $img_h ), $color );

}

for($i = 0; $i <= 10; ++ $i) {

         //设置直线颜色

         $color = imageColorAllocate ( $img, mt_rand ( 0, 255 ), mt_rand ( 0, 255 ), mt_rand ( 0, 255 ) );

         //在$img图像上随机画一条直线

         imageline ( $img, mt_rand ( 0, $img_w ), mt_rand ( 0, $img_h ), mt_rand ( 0, $img_w ), mt_rand ( 0, $img_h ), $color );

         //imagesetpixel($img,mt_rand(0,$img_w),mt_rand(0,$img_h),$color);

}

//加加框

$rect_color = imagecolorallocate ( $img, 0x90, 0x90, 0x90 );

imagerectangle ( $img, 0, 0, $img_w - 1, $img_h - 1, $rect_color );

$str_color = imagecolorallocate ( $img, mt_rand ( 0, 100 ), mt_rand ( 0, 100 ), mt_rand ( 0, 100 ) );

$font_w = imagefontwidth ( $font );

$font_h = imagefontheight ( $font );

$str_len = $font_w * $char_len;

imagestring ( $img, $font, ($img_w - $str_len) / 2, ($img_h - $font_h) / 2, $code, $str_color );