empty,isset,is_null comparison (1/4)_PHP tutorial

empty,isset,is_null I encountered some problems when using these functions. It even brings some security risks to your own programs. Many times, isset and empty are considered similar. Therefore, during development, I did not pay attention to it. When it was judged as a process, bug problems appeared.

1. Examples

a. A variable is not defined, how should we judge it?

view source print?

01

<?php

02	#不存在<font color="#aa7700">$test</font> 变量

03

04	<font color="#aa7700">$isset</font>= isset(<font color="#aa7700">$test</font>)?<font color="#0000ff">"test is define!"</font>:<font color="#0000ff">"test is undefine!"</font>;

05	<font color="#ff1493">echo</font> <font color="#0000ff">"isset:$issetrn"</font>;

06

07	<font color="#aa7700">$empty</font>=!<font color="#ff1493">empty</font>(<font color="#aa7700">$test</font>)?<font color="#0000ff">"test is define!"</font>:<font color="#0000ff">"test is undefine!"</font>;

08	<font color="#ff1493">echo</font> <font color="#0000ff">"empty:$emptyrn"</font>;

09

10	<font color="#aa7700">$is_null</font>=<font color="#ff1493">is_null</font>(<font color="#aa7700">$test</font>)?<font color="#0000ff">"test is define!"</font>:<font color="#0000ff">"test is undefine!"</font>;

11	<font color="#ff1493">echo</font> <font color="#0000ff">"is_null:$is_nullrn"</font>;

The test result is:

The result is out: empty and isset will first check whether the variable exists, and then detect the variable value. And is_null just checks the variable value directly to see if it is null, so if the variable is not defined, an error will occur!

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