//方法一:
function extend_1($file_name)
{
??? $retval="";
??? $pt=strrpos($file_name, ".");
??? if ($pt) $retval=substr($file_name, $pt+1, strlen($file_name) - $pt);
??? return ($retval);
}

//方法二
function extend_2($file_name)
{
??? $extend = pathinfo($file_name);
??? $extend = strtolower($extend["extension"]);
??? return $extend;
}

//方法三
function extend_3($file_name)
{
??? $extend =explode("." , $file_name);
??? return $extend[1];
}

$file = "c:\test.test\haha.haha.txt";

echo extend_1($file);
echo "\n
\n";
echo extend_2($file);
echo "\n
\n";
echo extend_3($file);

?>

返回结果：

txt
txt
test\haha

=============================================================

$file = "a.b.c.jpeg";
$ext = strrchr($file, ".");
print $ext;