比如http://www.csdn.net/
最后一个字符是/
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$str = "http://www.csdn.net/";
if(substr($str,-1) == "/")
$str .= "index.php";
用正则表达式，可写作
$str = "http://www.csdn.net/";
echo ereg_replace("/$","/index.php",$str);