php动态改变图片尺寸后输出，输出图片时使用下面的地址：
image_resize.php?img=image.jpg&w=150&h=150&constrain=1
w和h为要显示的尺寸
<?php header ("Content-type: image/jpeg");
/*
JPEG / PNG Image Resizer
Parameters (passed via URL):
img = path / url of jpeg or png image file
percent = if this is defined, image is resized by it's
value in percent (i.e. 50 to divide by 50 percent)
w = image width
h = image height
constrain = if this is parameter is passed and w and h are set
to a size value then the size of the resulting image
is constrained by whichever dimension is smaller
Requires the PHP GD Extension
Outputs the resulting image in JPEG Format
By: Michael John G. Lopez - www.sydel.net
Filename : imgsize.php
*/
$img = $_GET['img'];
$percent = $_GET['percent'];
$constrain = $_GET['constrain'];
$w = $_GET['w'];
$h = $_GET['h'];
// get image size of img
$x = @getimagesize($img);
// image width
$sw = $x[0];
// image height
$sh = $x[1];
if ($percent > 0) {
// calculate resized height and width if percent is defined
$percent = $percent * 0.01;
$w = $sw * $percent;
$h = $sh * $percent;
} else {
if (isset ($w) AND !isset ($h)) {
// autocompute height if only width is set
$h = (100 / ($sw / $w)) * .01;
$h = @round ($sh * $h);
} elseif (isset ($h) AND !isset ($w)) {
// autocompute width if only height is set
$w = (100 / ($sh / $h)) * .01;
$w = @round ($sw * $w);
} elseif (isset ($h) AND isset ($w) AND isset ($constrain)) {
// get the smaller resulting image dimension if both height
// and width are set and $constrain is also set
$hx = (100 / ($sw / $w)) * .01;
$hx = @round ($sh * $hx);
$wx = (100 / ($sh / $h)) * .01;
$wx = @round ($sw * $wx);
if ($hx
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