php如何解析url参数？

Question

I have a url

www.example.com?para1=&para2=2

I try to judge the para1 and para2 ;

isset($_GET['para1'] )       // true
 isset($_GET['para2'])       //true;
 isset($_GET['para3'])       // false

I think that isset($_GET['para1'] ) is false ,but it seems not .

My problem :

why isset($_GET['para1'] ) is true.How it parse the url in php?

Answer 1

A lot of people here are saying the value of a $_GET['para1'] is null but that is not the case. There is a comment on the php.net $_GET docs that provides a useful script to test this.

Given the URL: http://www.example.com?a

You can use this script to test results:

<?php
print_r($_GET);
if($_GET["a"] === "") echo "a is an empty string
";
if($_GET["a"] === false) echo "a is false
";
if($_GET["a"] === null) echo "a is null
";
if(isset($_GET["a"])) echo "a is set
";
if(!empty($_GET["a"])) echo "a is not empty";
?>
</pre>

The output would be:

a is an empty string
a is set

This is the case because a key without a value is actually an empty string not null

So to directly answer your question:

Even though it does not appear para1 has a value set, it actually does. The value is an empty string (""). Even though this value is falsy, it is still a value and as such, isset() returns true. You can use !empty() to return false on empty strings. See a comparison of isset() empty() and is_null() here.