header() が画像キャッシュを出力する方法の例

// put this above any php image generation code:
session_start(); 
header("Cache-Control: private, max-age=10800, pre-check=10800");
header("Pragma: private");
header("Expires: " . date(DATE_RFC822,strtotime(" 2 day")));

// the browser will send a $_SERVER['HTTP_IF_MODIFIED_SINCE'] if it has a cached copy 
if(isset($_SERVER['HTTP_IF_MODIFIED_SINCE'])){
  // if the browser has a cached version of this image, send 304
  header('Last-Modified: '.$_SERVER['HTTP_IF_MODIFIED_SINCE'],true,304);
  exit;
}

$img = "some_image.png";
if (isset($_SERVER['HTTP_IF_MODIFIED_SINCE']) && (strtotime($_SERVER['HTTP_IF_MODIFIED_SINCE']) == 
filemtime($img))) {
  // send the last mod time of the file back
  header('Last-Modified: '.gmdate('D, d M Y H:i:s', filemtime($img)).' GMT',true, 304);
  exit;
}

<?php
//调整图片大小
/**
 *图片按比例调整大小的原理：
 *1、比较原图大小是否小于等于目标大小，如果是则直接采用原图宽高
 *2、如果原图大小超过目标大小，则对比原图宽高大小
 *3、如：宽>高，则宽=目标宽, 高=目标宽的比例 * 原高
 *4、如：高>宽，则高=目标高，宽=目标高的比例 * 原宽   
**/
 
$image = "test.jpg";
$max_width = 200;
$max_height = 200;
 
$size = getimagesize($image);   //得到图像的大小
$width = $size[0];             
$height = $size[1];
 
$x_ratio = $max_width / $width;
$y_ratio = $max_height / $height;
 
if (($width <= $max_width) && ($height <= $max_height))
{
    $tn_width = $width;
    $tn_height = $height;
}
elseif (($x_ratio * $height) < $max_height)
{
    $tn_height = ceil($x_ratio * $height);
    $tn_width = $max_width;
}
else
{
    $tn_width = ceil($y_ratio * $width);
    $tn_height = $max_height;
}
 
$src = imagecreatefromjpeg($image);
$dst = imagecreatetruecolor($tn_width, $tn_height); //新建一个真彩色图像
imagecopyresampled($dst, $src, 0, 0, 0, 0,$tn_width, $tn_height, $width, $height);        //重采样拷贝部分图像并调整大小
header(&#39;Content-Type: image/jpeg&#39;);
imagejpeg($dst,null,100);
imagedestroy($src);
imagedestroy($dst);
?>