Lesson 4.4. isset vs empty vs is_null

How to test if a variable exists

In PHP, variables can provide from external sources. It is frequently usefull to check if the variable content is set or even if the variable exists or not before processing its value. The follwing table gather the main fonctions or tests and detailed their outputs:

Function	Do no exist	Empty	NULL	0	'' or ""	FALSE	Undefined	Other value
empty ($variable)	TRUE	TRUE	TRUE	TRUE	TRUE	TRUE	FALSE	FALSE
isset ($variable)	FALSE	FALSE	FALSE	FALSE	TRUE	TRUE	- *	TRUE
is_null ($variable)	TRUE	TRUE	TRUE	TRUE	FALSE	FALSE	FALSE	FALSE
$variable != NULL	FALSE	FALSE	FALSE	FALSE	FALSE	FALSE	TRUE	TRUE
$variable !== NULL	FALSE	FALSE	FALSE	FALSE	TRUE	TRUE	TRUE	TRUE
$variable != FALSE	FALSE	FALSE	FALSE	FALSE	FALSE	FALSE	TRUE	TRUE
$variable !== FALSE	TRUE	TRUE	TRUE	TRUE	TRUE	FALSE	TRUE	TRUE

* Calling the function isset(undefined) creates a PHP error.

Proof by example

See also

• Web development class
• Lesson 1.1. History of the Internet
• Lesson 1.2 Introduction to HTML
• Lesson 1.3. Special characters in HTML
• Lesson 1.4. Comments in HTML
• Lesson 1.5. Global structure of HTML pages
• Lesson 1.6. Hyperlinks in HTML
• Lesson 1.7. ID attribute in HTML tags
• Lesson 1.8. HTML anchors
• Lesson 1.9. Images in HTML pages
• Lesson 1.10. HTML tables
• Lesson 1.11. Most used HTML tags
• Lesson 1.12. Some advice
• Lesson 1.13. Ressources
• Lesson 2.1. Introduction to CSS
• Lesson 2.2. Inline CSS
• Lesson 2.3. Internal CSS
• Lesson 2.4. External CSS
• Lesson 2.5. CSS selectors
• Lesson 2.6. CSS colors
• Lesson 2.7. Margin and padding
• Lesson 2.8. CSS units
• Lesson 2.9. Most used CSS properties
• Lesson 3.1. History of JavaScript
• Lesson 3.2. JavaScript Hello World
• Lesson 3.3. Console.log
• Lesson 3.4. Comments in JavaScript
• Lesson 3.5. Variables in JavaScript
• Lesson 3.6. JavaScript events
• Lesson 3.7. Document Object Model (DOM)
• Lesson 3.8. Changing HTML
• Lesson 3.8. JavaScript frameworks
• Lesson 4.1. Introduction to PHP
• Lesson 4.2. PHP tags
• Lesson 4.3. PHP variables
• Lesson 4.5. PHP operators
• Lesson 4.6. PHP quotes
• Lesson 4.7. Tests in PHP: if .. else
• Lesson 4.8. PHP arrays
• Lesson 4.9. PHP loops
• Lesson 4.10. PHP sessions
• Lesson 5.1. HTML forms
• Lesson 5.2. HTML forms, POST or GET?
• Lesson 5.3. Input fields
• Lesson 5.4. HTML form fields
• Lesson 6.1. Introduction to MySQL
• Lesson 6.2. phpMyAdmin
• Lesson 6.3. SQL queries
• Lesson 6.4. Connect to MySQL in PHP
• Lesson 6.5. SQL queries in PHP
• Lesson 6.6. Getting data from MySQL in PHP
• Lesson 6.7. Get last ID

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