相信很多人都想过如何用PHP生成GIF动画来实现动态图片验证码，以下是实现过程。
ImageCode函数通过GIFEncoder类实现的GIF动画的PHP源代码，有兴趣的朋友可以研究一下。

效果如图：
复制代码 代码如下:
/**
 
* ImageCode 生成GIF图片验证
 
* @param $string 字符串
 
* @param $width 宽度
 
* @param $height 高度
 
* */
 
function ImageCode($string = '', $width = 75, $height = 25)
 
{
 
$authstr = $string "codetitle">复制代码 代码如下:
<"GIF89a";                /* GIF header 6 bytes        */
    var $VER = "GIFEncoder V2.06";        /* Encoder version                */
    var $BUF = Array ( );
    var $LOP =  0;
    var $DIS =  2;
    var $COL = -1;
    var $IMG = -1;
    var $ERR = Array (
    'ERR00' =>"Does not supported function for only one image!",
    'ERR01' =>"Source is not a GIF image!",
    'ERR02' =>"Unintelligible flag ",
    'ERR03' =>"Could not make animation from animated GIF source",
    );
    /*
    :::::::::::::::::::::::::::::::::::::::::::::::::::
    ::
    ::        GIFEncoder...
    ::
    */
    function GIFEncoder        (
    $GIF_src, $GIF_dly, $GIF_lop, $GIF_dis,
    $GIF_red, $GIF_grn, $GIF_blu, $GIF_mod
    ) {
        if ( ! is_array ( $GIF_src ) && ! is_array ( $GIF_tim ) ) {
            printf        ( "%s: %s", $this->VER, $this->ERR [ 'ERR00' ] );
            exit        ( 0 );
        }
        $this->LOP = ( $GIF_lop > -1 ) "url" ) {
                $this->BUF [ ] = fread ( fopen ( $GIF_src [ $i ], "rb" ), filesize ( $GIF_src [ $i ] ) );
            }
            else if ( strToLower ( $GIF_mod ) == "bin" ) {
                $this->BUF [ ] = $GIF_src [ $i ];
            }
            else {
                printf        ( "%s: %s ( %s )!", $this->VER, $this->ERR [ 'ERR02' ], $GIF_mod );
                exit        ( 0 );
            }
            if ( substr ( $this->BUF [ $i ], 0, 6 ) != "GIF87a" && substr ( $this->BUF [ $i ], 0, 6 ) != "GIF89a" ) {
                printf        ( "%s: %d %s", $this->VER, $i, $this->ERR [ 'ERR01' ] );
                exit        ( 0 );
            }
            for ( $j = ( 13 + 3 * ( 2 << ( ord ( $this->BUF [ $i ] { 10 } ) & 0x07 ) ) ), $k = TRUE; $k; $j++ ) {
                switch ( $this->BUF [ $i ] { $j } ) {
                    case "!":
                        if ( ( substr ( $this->BUF [ $i ], ( $j + 3 ), 8 ) ) == "NETSCAPE" ) {
                            printf        ( "%s: %s ( %s source )!", $this->VER, $this->ERR [ 'ERR03' ], ( $i + 1 ) );
                            exit        ( 0 );
                        }
                        break;
                    case ";":
                        $k = FALSE;
                        break;
                }
            }
        }
        GIFEncoder::GIFAddHeader ( );
        for ( $i = 0; $i < count ( $this->BUF ); $i++ ) {
            GIFEncoder::GIFAddFrames ( $i, $GIF_dly [ $i ] );
        }
        GIFEncoder::GIFAddFooter ( );
    }
    /*
    :::::::::::::::::::::::::::::::::::::::::::::::::::
    ::
    ::        GIFAddHeader...
    ::
    */
    function GIFAddHeader ( ) {
        $cmap = 0;
        if ( ord ( $this->BUF [ 0 ] { 10 } ) & 0x80 ) {
            $cmap = 3 * ( 2 << ( ord ( $this->BUF [ 0 ] { 10 } ) & 0x07 ) );
            $this->GIF .= substr ( $this->BUF [ 0 ], 6, 7                );
            $this->GIF .= substr ( $this->BUF [ 0 ], 13, $cmap        );
            $this->GIF .= "!/377/13NETSCAPE2.0/3/1" . GIFEncoder::GIFWord ( $this->LOP ) . "/0";
        }
    }
    /*
    :::::::::::::::::::::::::::::::::::::::::::::::::::
    ::
    ::        GIFAddFrames...
    ::
    */
    function GIFAddFrames ( $i, $d ) {
        $Locals_str = 13 + 3 * ( 2 << ( ord ( $this->BUF [ $i ] { 10 } ) & 0x07 ) );
        $Locals_end = strlen ( $this->BUF [ $i ] ) - $Locals_str - 1;
        $Locals_tmp = substr ( $this->BUF [ $i ], $Locals_str, $Locals_end );
        $Global_len = 2 << ( ord ( $this->BUF [ 0  ] { 10 } ) & 0x07 );
        $Locals_len = 2 << ( ord ( $this->BUF [ $i ] { 10 } ) & 0x07 );
        $Global_rgb = substr ( $this->BUF [ 0  ], 13,
        3 * ( 2 << ( ord ( $this->BUF [ 0  ] { 10 } ) & 0x07 ) ) );
        $Locals_rgb = substr ( $this->BUF [ $i ], 13,
        3 * ( 2 << ( ord ( $this->BUF [ $i ] { 10 } ) & 0x07 ) ) );
        $Locals_ext = "!/xF9/x04" . chr ( ( $this->DIS << 2 ) + 0 ) .
        chr ( ( $d 0 ) & 0xFF ) . chr ( ( $d 8 ) & 0xFF ) . "/x0/x0";
        if ( $this->COL > -1 && ord ( $this->BUF [ $i ] { 10 } ) & 0x80 ) {
            for ( $j = 0; $j < ( 2 << ( ord ( $this->BUF [ $i ] { 10 } ) & 0x07 ) ); $j++ ) {
                if        (
                ord ( $Locals_rgb { 3 * $j + 0 } ) == ( $this->COL   0 ) & 0xFF &
                ord ( $Locals_rgb { 3 * $j + 1 } ) == ( $this->COL   8 ) & 0xFF &
                ord ( $Locals_rgb { 3 * $j + 2 } ) == ( $this->COL 16 ) & 0xFF
                ) {
                    $Locals_ext = "!/xF9/x04" . chr ( ( $this->DIS << 2 ) + 1 ) .
                    chr ( ( $d 0 ) & 0xFF ) . chr ( ( $d 8 ) & 0xFF ) . chr ( $j ) . "/x0";
                    break;
                }
            }
        }
        switch ( $Locals_tmp { 0 } ) {
            case "!":
                $Locals_img = substr ( $Locals_tmp, 8, 10 );
                $Locals_tmp = substr ( $Locals_tmp, 18, strlen ( $Locals_tmp ) - 18 );
                break;
            case ",":
                $Locals_img = substr ( $Locals_tmp, 0, 10 );
                $Locals_tmp = substr ( $Locals_tmp, 10, strlen ( $Locals_tmp ) - 10 );
                break;
        }
        if ( ord ( $this->BUF [ $i ] { 10 } ) & 0x80 && $this->IMG > -1 ) {
            if ( $Global_len == $Locals_len ) {
                if ( GIFEncoder::GIFBlockCompare ( $Global_rgb, $Locals_rgb, $Global_len ) ) {
                    $this->GIF .= ( $Locals_ext . $Locals_img . $Locals_tmp );
                }
                else {
                    $byte  = ord ( $Locals_img { 9 } );
                    $byte |= 0x80;
                    $byte &= 0xF8;
                    $byte |= ( ord ( $this->BUF [ 0 ] { 10 } ) & 0x07 );
                    $Locals_img { 9 } = chr ( $byte );
                    $this->GIF .= ( $Locals_ext . $Locals_img . $Locals_rgb . $Locals_tmp );
                }
            }
            else {
                $byte  = ord ( $Locals_img { 9 } );
                $byte |= 0x80;
                $byte &= 0xF8;
                $byte |= ( ord ( $this->BUF [ $i ] { 10 } ) & 0x07 );
                $Locals_img { 9 } = chr ( $byte );
                $this->GIF .= ( $Locals_ext . $Locals_img . $Locals_rgb . $Locals_tmp );
            }
        }
        else {
            $this->GIF .= ( $Locals_ext . $Locals_img . $Locals_tmp );
        }
        $this->IMG  = 1;
    }
    /*
    :::::::::::::::::::::::::::::::::::::::::::::::::::
    ::
    ::        GIFAddFooter...
    ::
    */
    function GIFAddFooter ( ) {
        $this->GIF .= ";";
    }
    /*
    :::::::::::::::::::::::::::::::::::::::::::::::::::
    ::
    ::        GIFBlockCompare...
    ::
    */
    function GIFBlockCompare ( $GlobalBlock, $LocalBlock, $Len ) {
        for ( $i = 0; $i < $Len; $i++ ) {
            if        (
            $GlobalBlock { 3 * $i + 0 } != $LocalBlock { 3 * $i + 0 } ||
            $GlobalBlock { 3 * $i + 1 } != $LocalBlock { 3 * $i + 1 } ||
            $GlobalBlock { 3 * $i + 2 } != $LocalBlock { 3 * $i + 2 }
            ) {
                return ( 0 );
            }
        }
        return ( 1 );
    }
    /*
    :::::::::::::::::::::::::::::::::::::::::::::::::::
    ::
    ::        GIFWord...
    ::
    */
    function GIFWord ( $int ) {
        return ( chr ( $int & 0xFF ) . chr ( ( $int 8 ) & 0xFF ) );
    }
    /*
    :::::::::::::::::::::::::::::::::::::::::::::::::::
    ::
    ::        GetAnimation...
    ::
    */
    function GetAnimation ( ) {
        return ( $this->GIF );
    }
}
?>