Base64 编码介绍、Base64编码转换原理与算法

/**
 *base64编码方法、本方法只是做base64转换过程代码举例说明，通过该例子可以任意改造不同语言版
 *@author 程默
 *@copyright http://blog.chacuo.net
 *@param $src 原字符串
 *@return string base64字符串*
 */
function c_base64_encode($src)
{
	static $base="ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/";
	
	////将原始的3个字节转换为4个字节
	$slen=strlen($src);
	$smod = ($slen%3);
	$snum = floor($slen/3);
	
	
	$desc = array();
	
	for($i=0;$i<$snum;$i++)
	{
		////读取3个字节
		$_arr = array_map('ord',str_split(substr($src,$i*3,3)));
		
		///计算每一个base64值
		$_dec0= $_arr[0]>>2;
		$_dec1= (($_arr[0]&3)<<4)|($_arr[1]>>4);
		$_dec2= (($_arr[1]&0xF)<<2)|($_arr[2]>>6);	
		$_dec3= $_arr[2]&63;

		$desc = array_merge($desc,array($base[$_dec0],$base[$_dec1],$base[$_dec2],$base[$_dec3]));
	}
	
	if($smod==0) return implode('',$desc);
	
	///计算非3倍数字节
	$_arr = array_map('ord',str_split(substr($src,$snum*3,3)));
	$_dec0= $_arr[0]>>2;
	///只有一个字节
	if(!isset($_arr[1]))
	{
		$_dec1= (($_arr[0]&3)<<4);
		$_dec2=$_dec3="=";
	}
	else 
	{
		///2个字节
		$_dec1= (($_arr[0]&3)<<4)|($_arr[1]>>4);
		$_dec2= $base[($_arr[1]&7)<<2];
		$_dec3="=";
	}
	$desc = array_merge($desc,array($base[$_dec0],$base[$_dec1],$_dec2,$_dec3));
	return implode('',$desc);
}