How to convert 32 bit jpg images to 24 bit image using php and without imagemagick

Question

I have code which creates jpg image from hexadecimal string. The generated jpg image having 32 bit depth but i want to make it 24 bit. Is it possible to change the bit depth without imagemagick ??

Here is the code to generate JPG Img :

$hexstring = XXXXXXX;
    if(is_string($hexstring) && !empty($hexstring))
    {
            $hex='';
            for($a=0; $a<strlen($hexstring); $a+=2)
            {
                    $hex.=chr(hexdec($hexstring{$a}.$hexstring{($a+1)}));
            }

            $fileHandle= fopen($imgFile, 'w+'); 
            fwrite($fileHandle,$hex); 
            fclose($fileHandle);
    }

Thanks