Difference Between isset and empty in PHP

Understanding the difference between isset() and empty() is crucial for writing clean and bug-free PHP code. While both functions are used to check the state of variables, they behave differently when handling null, empty strings, zeros, and undefined variables. This article provides a clear comparison of how isset() and empty() evaluate various values, helping you choose the right function for your conditional checks.

What is isset()?

The isset() function determines whether a variable is declared and is not null.

It returns true if the variable exists and has any value other than null (including 0, empty strings, or false).
It returns false if the variable does not exist, or if it is explicitly set to null.

$var = "";
var_dump(isset($var)); // Returns true

$var2 = null;
var_dump(isset($var2)); // Returns false

What is empty()?

The empty() function determines whether a variable is empty. A variable is considered empty if it does not exist, or if its value evaluates to false.

It returns true if the variable is not set, or if its value is one of the following:
- "" (an empty string)
- 0 (integer 0)
- 0.0 (float 0)
- "0" (string containing the character 0)
- null
- false
- [] (an empty array)
It returns false if the variable exists and contains a non-empty value (like a non-zero number or a non-empty string).

$var = "";
var_dump(empty($var)); // Returns true

$var2 = "Hello";
var_dump(empty($var2)); // Returns false

Comparison Table

The table below illustrates how isset() and empty() evaluate different variable states:

Variable Value	isset() Output	empty() Output
Undefined variable	`false`	`true`
`null`	`false`	`true`
`""` (empty string)	`true`	`true`
`0` (integer)	`true`	`true`
`"0"` (string)	`true`	`true`
`[]` (empty array)	`true`	`true`
`false`	`true`	`true`
`true`	`true`	`false`
`"hello"` (string)	`true`	`false`

When to Use Which?

Use isset() when you want to verify if a variable or an array key exists, regardless of whether its content is “falsy.” This is ideal for checking if form checkboxes were submitted or if specific optional array configuration parameters are present.
Use empty() when you want to ensure a variable contains meaningful data before using it. This is useful for validating required text inputs from user forms, where spaces or empty strings should be treated as missing information.