"PHP" uses isset to detect whether a variable exists ____php

In the learning of PHP, we may encounter variables that do not exist, have long been unset or null, and are notice when we refer to them. Well, we can determine whether they exist before referencing them.

The answer is certainly also simple.

knowledge Point: Use Isset to detect whether a variable exists.

Example 1:

<?php
$a =1;
$b;
if (Isset ($a)) {
    echo ' ok! ', ' <br/> ';
} else{
    Echo ' no! ', ' <br/> ';
}
if (Isset ($b)) {
    echo ' ok! ', ' <br/> ';
} else{
    Echo ' no! ', ' <br/> ';
>

The output is:

ok!
No!