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PHP Identifiers

By Shahnail Khan Last updated : December 10, 2023

PHP identifiers play a crucial role in coding with PHP, enabling developers to name variables, functions, classes, and other entities in their code. In this tutorial, we will cover the fundamentals of PHP identifiers, rules for PHP Identifiers, and discuss some best practices to ensure your code is clean and maintainable.

What are PHP Identifiers?

PHP identifiers are names given to various elements in your code, such as variables, functions, classes, and constants. These names are used to reference and manipulate these entities throughout your PHP scripts.

Rules for PHP Identifiers

1. Start with a Letter or Underscore

Identifiers must begin with a letter (a-z, A-Z) or an underscore (_).

2. Followed by Letters, Numbers, or Underscores

After the initial letter or underscore, you can use letters, numbers, or additional underscores.

3. Case-Sensitivity

PHP identifiers are case-sensitive, meaning $variable and $Variable would be considered two different identifiers.

4. Reserved Words

Avoid using PHP reserved words (e.g., echo, if, else) as identifiers.

5. Length Limitation

While there is no strict limit, it's recommended to keep identifiers reasonably short and meaningful for better code readability.

PHP Identifiers Examples

The below are the some of the examples of valid and invalid identifiers:

Valid Identifiers

The valid identifiers are those identifiers that follow all the rules of creating an identifier. Some of the valid identifiers are:


Invalid Identifiers

The invalid identifiers are those identifiers that do not follow all the rules of creating an identifier. Some of the invalid identifiers are:

$123invalid;       // starts with a number
$invalid-identifier; // contains a hyphen
$abstract;    // uses a reserved word

Best Practices for Identifiers

1. Descriptive and Meaningful

Choose names that convey the purpose of the variable, function, or class.

2. CamelCase for Variables and Functions

Use camelCase (e.g., $myVariable, calculateTotal()) for variables and function names.

3. PascalCase for Classes

Use PascalCase (e.g., MyClass) for class names.

4. Underscores for Constants

Use uppercase letters with underscores (e.g., MAX_SIZE) for constants.

5. Consistency

Maintain a consistent naming convention throughout your codebase.


Understanding and following the rules and best practices for identifiers is essential for writing clean, readable, and maintainable code. By adhering to these guidelines, you'll improve collaboration with other developers and enhance the overall quality of your PHP projects.

PHP identifiers Exercise

Determine whether the following examples are valid PHP identifiers according to the rules discussed in the tutorial. If an identifier is valid, explain why; if it's invalid, specify the rule it doesn't follow.

  1. $myVariable
  2. $_user_name
  3. $counter123
  4. $123invalid
  5. $invalid-identifier
  6. $reserved_word
  7. $CamelCase
  8. 123variable
  9. my_variable
  10. $ThisIsAValidIdentifier


  1. Valid; Follows the rule of starting with a letter.
  2. Valid; Starts with an underscore, followed by letters and underscores.
  3. Valid; Complies with the rules, using letters and numbers after the initial letter.
  4. Invalid; Starts with a number, doesn’t follow the rule.
  5. Invalid; Contains a hyphen, which is not allowed.
  6. Invalid; Uses a reserved word as an identifier.
  7. Invalid; Case-sensitive; should be $camelCase for consistency.
  8. Invalid; Starts with a number.
  9. Valid; Uses underscores, numbers, and letters, following the rules.
  10. Valid; Follows the rules and is case-sensitive.

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