Detailed introduction to the differences between is_null, empty, isset, and unset in php_PHP tutorial

is_null, empty, isset, unset Let’s take a look at the description of these four functions first.

isset determines whether the variable already exists (configuration)
unset deletes (releases) the variable
empty determines whether the variable is empty
is_null determines whether the variable is NULL
ok, people have already started . So to start, apart from unset, the other three of these four functions are judgment functions. Unset is the first to go out because it can't make mistakes. The second is is_null. We can think of it as !isset, which is an inverse operation of isset. , the following table can clearly illustrate the relationship between them:

Copy code The code is as follows:

Variables                                            empty is_null isset
$a=”” true
$a=null true true false
var $a true true false
$a=array() true false                true
$a= true false true
$a=0 true
$a="0" true
$ a="true"                                                                                                                                                                                                                                                                                                    a="true

From this we can find that as long as the variable is "" or 0, or false and null, empty will return true as long as these values ​​​​are, and isset determines whether the variable exists, as long as your variable is not null or not Assignment and return results are all true, and is_null is the opposite result of isset.

Of course, if you just want to do the following:
echo !isset($_GET['a']); //If you can’t get the value of variable a
echo empty($_GET['a'] ); //If the value of variable a is empty

Then the result is the same and can be used.

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