1. 返回文件扩展名
function getformat($file)
{
$ext=strrchr($file,".");
$format=strtolower($ext);
return $format;
}

2.格式化变量
<?
$num = 1;
printf("%04d", $num);
?>

3.php重定向网页
// 例如重定向到www.cgsir.com (注意重定向之前不要有html内容)
header("location://www.yx10011.com");
或
echo "<meta http-equiv="refresh" content="0;url=//www.yx10011.com.com">";

4.限制上传的文件大小
//$limit_size为限制最大文件大小
$limit_size=50000;
$file_size=$HTTP_POST_FILES["ufile"]["size"];
if($file_size >= $limit_size) {
echo "你的文件超过的限制的大小<BR>";
echo "你的文件大小为= ".$file_size;
echo " K";
echo "<BR>文件大小限制为= 50000 k";
}
else {
// 上传到什么目录,也就是从临时目录拷贝到目标目录
if(copy($HTTP_POST_FILES["ufile"]["tmp_name"], $path))
{
echo "上传成功<BR/>";
echo "<img src="$path" width="150" height="150">";
}

5.php常用的对字符串进行加密的算法：
5.1 $db_password = md5($random_password);
5.2 $db_password = sh1($random_password);

6.退出登录
// 退出登录
session_start();
include_once("includes/header.php");
if (isset($_SESSION["user_id"]))
{
unset($_SESSION["user_id"]);
session_destroy();

echo "<div align="center">";
echo "<span class="STYLE1">成功退出！</span><br />";
echo "<p><span class="STYLE1">正在跳转，请稍等......</span></p>";
echo "<script language="javascript">";
echo "function Jump()";
echo "{ ";
echo " parent.location.href="index.php" ";
echo "} " ;
echo "document.onload = setTimeout("Jump()" , 2 * 1000)";
echo "</script>";
echo "<span class="STYLE1"><a href="index.php">直接返回</a></span><br /><br />";
echo "</div>";
exit(0);
}
else
{
echo "<span class="STYLE1">您还没有登录呢！</span>";
}

include_once("includes/footer.php");
?>