php图片显示一片乱码怎么办

<?php
header("Content-Type:image/jpg");//图片编码设置

function setWater($dst_path, $save_path, $text)
{
    // 创建图片的实例
    $dst = imagecreatefromstring(file_get_contents($dst_path));

    // 文字样式
    $font    = realpath('./font/arial.ttf');
    $black    = imagecolorallocate($dst, 255, 255, 255);    //字体颜色

    // 加文字
    imagefttext($dst, 22, 0, 20, 40, $black, $font, $text);

    // 输出图片
    list($dst_w, $dst_h, $dst_type) = getimagesize($dst_path);

    switch ($dst_type) {
        //GIF
        case 1:
            //header('Content-Type: image/gif');
            $save_path = $save_path . $text .'.gif';
            imagegif($dst, $save_path, 90);
            break;
        //JPG
        case 2:
            //header('Content-Type: image/jpeg');
            $save_path = $save_path . $text .'.jpg';
            imagejpeg($dst, $save_path, 90);
            break;
        //PNG
        case 3:
            //header('Content-Type: image/png');
            $save_path = $save_path . $text .'.png';
            imagepng($dst, $save_path, 90);
            break;
        default:
            break;
    }

    imagedestroy($dst);

}

$dst_path = '7002.jpg';
//$text     = '7001';
//setWater($dst_path, './tmppic/', $text);

for ($i=1; $i<=240; $i++) {
    $text = '00'. $i;
    $text = substr($text, -3);
    
    setWater($dst_path, './tmppic/', $text);
}

echo('ok');