imagecreatefrombmp用PHP讀取BMP

<?php
    function imagecreatefrombmp($p_sFile)
    {
        //    Load the image into a string
        $file    =    fopen($p_sFile,"rb");
        $read    =    fread($file,10);
        while(!feof($file)&&($read<>""))
            $read    .=    fread($file,1024);
       
        $temp    =    unpack("H*",$read);
        $hex    =    $temp[1];
        $header    =    substr($hex,0,108);
        //    Process the header
        //    Structure: http://www.fastgraph.com/help/bmp_header_format.html
        if (substr($header,0,4)=="424d")
        {
            //    Cut it in parts of 2 bytes
            $header_parts    =    str_split($header,2);
            //    Get the width        4 bytes
            $width            =    hexdec($header_parts[19].$header_parts[18]);
            //    Get the height        4 bytes
            $height            =    hexdec($header_parts[23].$header_parts[22]);
            //    Unset the header params
            unset($header_parts);
        }
        //    Define starting X and Y
        $x                =    0;
        $y                =    1;
        //    Create newimage
        $image            =    imagecreatetruecolor($width,$height);
        //    Grab the body from the image
        $body            =    substr($hex,108);
        //    Calculate if padding at the end-line is needed
        //    Divided by two to keep overview.
        //    1 byte = 2 HEX-chars
        $body_size        =    (strlen($body)/2);
        $header_size    =    ($width*$height);
        //    Use end-line padding? Only when needed
        $usePadding        =    ($body_size>($header_size*3)+4);
        //    Using a for-loop with index-calculation instaid of str_split to avoid large memory consumption
        //    Calculate the next DWORD-position in the body
        for ($i=0;$i<$body_size;$i+=3)
        {
            //    Calculate line-ending and padding
            if ($x>=$width) {
                //    If padding needed, ignore image-padding
                //    Shift i to the ending of the current 32-bit-block
                if ($usePadding)
                    $i    +=    $width%4;
                //    Reset horizontal position
                $x    =    0;
                //    Raise the height-position (bottom-up)
                $y++;
                //    Reached the image-height? Break the for-loop
                if ($y>$height)
                    break;
            }
            //    Calculation of the RGB-pixel (defined as BGR in image-data)
            //    Define $i_pos as absolute position in the body
            $i_pos    =    $i*2;
            $r        =    hexdec($body[$i_pos+4].$body[$i_pos+5]);
            $g        =    hexdec($body[$i_pos+2].$body[$i_pos+3]);
            $b        =    hexdec($body[$i_pos].$body[$i_pos+1]);
            $gray     =    (int)($r + $g + $b)/3;
            //    Calculate and draw the pixel
            $color    =    imagecolorallocate($image,$gray,$gray,$gray);
            imagesetpixel($image,$x,$height-$y,$color);
            //    Raise the horizontal position
            $x++;
        }
        //    Unset the body / free the memory
        unset($body);
        //    Return image-object
        return $image;
    }