Misunderstanding isset() behavior in PHP

The isset() function in PHP is often misunderstood, leading to errors and unintended outcomes in code. In this article, we will delve deeper into the workings of isset() and show you how to use it properly.

isset() is used in PHP to determine if a variable has been defined and holds a value that is not NULL. If the variable meets these conditions, isset() will return TRUE. If not, it returns FALSE.

It is important to remember that isset() only checks if a variable has been defined and has a value that is not NULL. It does not check for the existence of an array key or object property. For instance:

$array = array();
if (isset($array['key'])) {
   // this will not be executed
}

In the example above, isset($array['key']) returns FALSE, even though the array $array exists. To verify the existence of an array key, you should use the array_key_exists() function instead:

$array = array();
if (array_key_exists('key', $array)) {
   // this will be executed
}

Another common error with isset() is the assumption that it will return TRUE for variables with a value of 0 or an empty string. However, 0 and an empty string are valid values and isset() will return TRUE for these variables:

$var = 0;
if (isset($var)) {
   // this will be executed
}

$var = '';
if (isset($var)) {
   // this will be executed
}

In conclusion, having a clear understanding of the behavior of isset() in PHP is essential for writing efficient and error-free code. It is important to be mindful of what isset() does and does not check for and to use it appropriately to avoid any unexpected results.