什么使变量存在PHP？

Question

I've been looking at the documentation for isset() on PHP's website at https://secure.php.net/manual/en/function.isset.php. The return value for this function is apparently as follows:

Returns TRUE if var exists and has value other than NULL. FALSE otherwise.

What is the difference between a variable which is set to NULL and a variable that is not set? Is it true that if $variable = NULL; isset($variable) would return FALSE, meaning the variable is NOT set?

EDIT: How would I then be able to distinguish between a variable whose value is NULL and a variable which does not exist?

Answer 1

What is the difference between a variable which is set to NULL and a variable that is not set?

Take a look at PHP types comparison table #1 (Comparisons of $x with PHP functions). The lines #2-#4 read:

| Expression      | gettype() | empty() | is_null() | isset() | if($x) |
|-----------------+-----------+---------+-----------+---------+--------|
| $x = null;      | NULL      | TRUE    | TRUE      | FALSE   | FALSE  |
| var $x;         | NULL      | TRUE    | TRUE      | FALSE   | FALSE  |
| $x is undefined | NULL      | TRUE    | TRUE      | FALSE   | FALSE  |

Can you spot any difference between them? Me neither.

There is one difference, though. When a variable $x is undefined, gettype($x), is_null(), if($x) and any code that attempts to get its value triggers a notice ("Notice: Undefined variable: x"). empty() and isset() do not trigger any error, warning or notice when their argument does not exist because their purpose is to classify the presence and value of their argument.

How would I then be able to distinguish between a variable whose value is NULL and a variable which does not exist?

Apart from the notice triggered as explained above, there is no difference between a variable whose value is NULL and a variable that doesn't exist. Even the notice is just a hint about a possible error in the code or a way to improve its readability but with or without it, the behaviour is the same.

I cannot figure out any practical usage of telling apart an undefined variable from a variable that stores NULL.

What makes a variable exist PHP?

A variable exist in PHP since it was assigned a value for the first time until it is unset() or until the code exits its scope (whichever comes first).