`if（isset（$ _ SESSION））`和`if（$ _SESSION）`之间的区别？

Question

I've noticed that frequently people simply write

<?php  if($_SESSION['username']) {...} ?>

while I have been using:

 <?php if(isset($_SESSION['username'])) {...} ?> 

Could someone explain the difference when checking if a variable is set (that's what I'd be using it for)?

Answer 1

According to PHP.net, isset() does the following:

Determine if a variable is set and is not NULL.

When writing:

<?php  if($_SESSION['username']) {...} ?>

You are checking to see if $_SESSION['username'] is equal to true. In other words, you are checking if the value does not equal false.

According to PHP.net the following are considered FALSE:

When converting to boolean, the following values are considered FALSE:

the boolean FALSE itself
the integer 0 (zero)
the float 0.0 (zero)
the empty string, and the string "0"
an array with zero elements
an object with zero member variables (PHP 4 only)
the special type NULL (including unset variables)
SimpleXML objects created from empty tags

As you can see unset variables / NULL variables are considered FALSE. Therefore by testing if the $_SESSION element is true, you are also determining if it exists.

Isset, on the otherhand, actually checks if the variable exists. If you would like to know if there is a SESSION variable by that name, use isset() as testing it for TRUE/FALSE isn't dependent on whether the variable exists or not.

Further, look at the following examples:

$_SESSION['a'] = FALSE;
if($_SESSION['a']){
echo 'Hello'; //This line is NOT echo'd.
}

if(isset($_SESSION['b'])){
echo 'Hello'; //This line is NOT echo'd because $_SESSION['b'] has not been set.
}