如何利用php把上传的图片压缩？

PHP压缩图片两个步骤：
1、添加类

//php生成缩略图片的类

class ResizeImage{

    public $type;//图片类型

    public $width;//实际宽度

    public $height;//实际高度

    public $resize_width;//改变后的宽度

    public $resize_height;//改变后的高度

    public $cut;//是否裁图

    public $srcimg;//源图象 

    public $dstimg;//目标图象地址

    public $im;//临时创建的图象

    public $quality;//图片质量

    public $img_array=array('jpg','png','gif');

    //http://www.phpernote.com/php-function/782.html

    function __construct($img,$wid,$hei,$c,$dstpath,$quality=100){

        $this->srcimg=$img;

        $this->resize_width=$wid;

        $this->resize_height=$hei;

        $this->cut=$c;

        $this->quality=$quality;

        //$this->type=strtolower(substr(strrchr($this->srcimg,'.'),1));//图片的类型

        $this->type=$this->checkFileType($this->srcimg);//更为严格的检测图片类型

        if(!in_array($this->type,$this->img_array)){

            return '';

        }

        $this->initi_img();//初始化图象

        $this -> dst_img($dstpath);//目标图象地址

        $this->width=imagesx($this->im);

        $this->height=imagesy($this->im);

        $this->newimg();//生成图象

        ImageDestroy($this->im);

    }

    function newimg(){

        $resize_ratio=($this->resize_width)/($this->resize_height);//改变后的图象的比例

        $ratio=($this->width)/($this->height);//实际图象的比例

        if(($this->cut)=='1'){//裁图

            if(function_exists('imagepng')&&(str_replace('.','',PHP_VERSION)>=512)){//针对php版本大于5.12参数变化后的处理情况

                $quality=9;

            }

            if($ratio>=$resize_ratio){//高度优先

                $newimg=imagecreatetruecolor($this->resize_width,$this->resize_height);

                //上色 
                $color=imagecolorallocate($newimg,255,255,255); 
                //设置透明 
                imagecolortransparent($newimg,$color); 
                imagefill($newimg,0,0,$color); 

                imagecopyresampled($newimg,$this->im,0,0,0,0,$this->resize_width,$this->resize_height,(($this->height)*$resize_ratio),$this->height);

                imagejpeg($newimg,$this->dstimg,$this->quality);

            }

            if($ratio<$resize_ratio){//宽度优先

                $newimg=imagecreatetruecolor($this->resize_width,$this->resize_height);

                //上色 
                $color=imagecolorallocate($newimg,255,255,255); 
                //设置透明 
                imagecolortransparent($newimg,$color); 
                imagefill($newimg,0,0,$color); 

                imagecopyresampled($newimg,$this->im,0,0,0,0,$this->resize_width,$this->resize_height,$this->width,(($this->width)/$resize_ratio));

                imagejpeg($newimg,$this->dstimg,$this->quality);

            }

        }else{//不裁图

            if($ratio>=$resize_ratio){

                $newimg=imagecreatetruecolor($this->resize_width,($this->resize_width)/$ratio);

                //上色 
                $color=imagecolorallocate($newimg,255,255,255); 
                //设置透明 
                imagecolortransparent($newimg,$color); 
                imagefill($newimg,0,0,$color); 

                imagecopyresampled($newimg,$this->im,0,0,0,0,$this->resize_width,($this->resize_width)/$ratio,$this->width,$this->height);

                imagejpeg($newimg,$this->dstimg,$this->quality);

            }

            if($ratio<$resize_ratio){

                $newimg=imagecreatetruecolor(($this->resize_height)*$ratio,$this->resize_height);

                //上色 
                $color=imagecolorallocate($newimg,255,255,255); 
                //设置透明 
                imagecolortransparent($newimg,$color); 
                imagefill($newimg,0,0,$color); 

                imagecopyresampled($newimg,$this->im,0,0,0,0,($this->resize_height)*$ratio,$this->resize_height,$this->width,$this->height);

                imagejpeg($newimg,$this->dstimg,$this->quality);

            }

        }

    }

    function initi_img(){//初始化图象

        if($this->type=='jpg'){

            $this->im=imagecreatefromjpeg($this->srcimg);

        }

        if($this->type=='gif'){

            $this->im=imagecreatefromgif($this->srcimg);

        }

        if($this->type=='png'){

            $this->im=imagecreatefrompng($this->srcimg);

        }

    }

    function dst_img($dstpath){//图象目标地址

        $full_length=strlen($this->srcimg);

        $type_length=strlen($this->type);

        $name_length=$full_length-$type_length;

        $name=substr($this->srcimg,0,$name_length-1);

        $this->dstimg=$dstpath;

        //echo $this->dstimg;

    }

    //读取文件前几个字节 判断文件类型

    function checkFileType($filename){

        $file=fopen($filename,'rb');

        $bin=fread($file,2); //只读2字节

        fclose($file);

        $strInfo =@unpack("c2chars",$bin);

        $typeCode=intval($strInfo['chars1'].$strInfo['chars2']);

        $fileType='';

        switch($typeCode){

            case 7790:

                $fileType='exe';

            break;

            case 7784:

                $fileType='midi';

            break;

            case 8297:

                $fileType='rar';

            break;

            case 255216:

                $fileType='jpg';

            break;

            case 7173:

                $fileType='gif';

            break;

            case 6677:

                $fileType='bmp';

            break;

            case 13780:

                $fileType='png';

            break;

            default:

                $fileType='unknown'.$typeCode;

            break;

        }

        if($strInfo['chars1']=='-1'&&$strInfo['chars2']=='-40'){

            return 'jpg';

        }

        if($strInfo['chars1']=='-119'&&$strInfo['chars2']=='80'){

            return 'png';

        }

        return $fileType;

    }

}

2、压缩图片

$resizeimage=new ResizeImage($file_path,300,169,0,$newname);

ResizeImage会将你正在上传的图片压缩至相应尺寸，同时上传。参数意义(路径, width, height, 是否截图, 图片名称)，$file_path的根目录是这个php所在的文件夹，可以修改调整至相应路径。
通过该方法，成功将900+kb的图片压缩至40+kb，图片加载迅速很多。