<?php
//Get jpeg image contents
ob_start();
imagejpeg($image, "", 100);
$image = ob_get_contents();
ob_end_clean();
//Or read jpeg image
$image = file_get_contents($file);
//Replace header image info to set DPI as units and the density for X and Y
//First parameter are units for X and Y densities (0x00 No units, 0x01 DPI, 0x02 DPC)
//Second parameter is the X density
//Third parameter is the Y density
$image = substr_replace($image, pack("Cnn", 0x01, 300, 300), 13, 5); // 預設72DPI改成300DPI
header("Content-type: image/jpeg");
header('Content-Disposition: attachment; filename="'.basename($file).'"');
echo $image;
?>
<?php
$filename= 'img/example.png';
// ob_start();
// $im = imagecreatefrompng($filename);
// imagepng($im);
// $file = ob_get_contents();
// ob_end_clean();
$file= file_get_contents($filename);
//数据块长度为9
$len= pack("N", 9);
//数据块类型标志为pHYs
$sign= pack("A*", "pHYs");
$sign= pack("A*", "pHYs");
//X方向和Y方向的分辨率均为300DPI（1像素/英寸=39.37像素/米），单位为米（0为未知，1为米）
$data= pack("NNC", 300 * 39.37, 300 * 39.37, 0x01);
//CRC检验码由数据块符号和数据域计算得到
$checksum= pack("N", crc32($sign. $data));
$phys= $len. $sign. $data. $checksum;
$pos= strpos($file, "pHYs");
if($pos> 0) {
//修改pHYs数据块
$file= substr_replace($file, $phys, $pos- 4, 21);
} else{
//IHDR结束位置（PNG头固定长度为8，IHDR固定长度为25）
$pos= 33;
//将pHYs数据块插入到IHDR之后
$file= substr_replace($file, $phys, $pos, 0);
}
header("Content-type: image/png");
header('Content-Disposition: attachment; filename="'. basename($filename) . '"');
echo$file;
?>
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