Case Sensitivity

Case sensitivity refers to the way PHP retrieves names when it looks for a definition: is it in a case-sensitive way, or case insensitive.

The case insensitive named structures are:

• Classes

• Enumerations

• Functions

• Interfaces

• Magic methods

• Magic constants

• Named parameters

• Methods

• Namespaces

• PHP keywords

• Traits

The case sensitive named structures are:

• Class constants

• Global constants, since PHP 8.0

• Array index

• Properties

• Variables

Case sensitivity has the potential to speed up PHP execution: the gain is a micro-optimisation, that requires millions of calls before yielding significant speed up.

The different case sensitivities in the language is balanced by the common practice to use the original case when using a structure.

<?php

    // two distinct variables
    $x = 1;
    $X = 2;

    // two identical calls
    foo();
    FOO();

    // Calling a class with its definition case
    $db = new SQLite3();
    // This is valid, and often reported by tools and conventions
    $db = new SQLite3();

?>

See also Unraveling the Quest for the Fastest Case Insensitive Char Comparison in PHP.

Related : Name, String, Name Conventions