`define()` vs. `const` in PHP: When Should I Use Which?

PHP: Contrasting define() and const

In PHP, you have the option of declaring constants via either the define keyword or the const keyword. While both methods share the purpose of constant declaration, there are several key differences between them that dictate their usage scenarios.

Define Keyword

The define keyword simplifies constant declaration and enables defining constants during runtime.

Const Keyword

In contrast, the const keyword facilitates constant declaration during compile time, limiting its scope to the compilation phase.

Disadvantages of const

• Conditional Restrictions: const does not allow conditional constant definition, requiring global constant declaration within the outermost scope.
• Expression Limitations: const only permits static scalar values, while define() supports any expression.
• Expression Syntax: define() grants the flexibility to use any expression as the constant name, including dynamic construction.
• Case Sensitivity: const enforces case sensitivity, whereas define() allows for case-insensitive constant definition (deprecated in PHP 7.3.0 and removed in PHP 8.0.0).

Advantages of const

• Enhanced readability: const presents a cleaner syntax compared to define(), adhering to the language construct style.
• Tool compatibility: Automated tools can statically analyze const-defined constants.
• Namespace distinction: const defines constants within the current namespace, while define() requires specifying the full namespace.
• Array support: const permits array constant declaration (PHP 5.6 onwards), whereas define() lacks array support.
• Class and interface constants: const can define class and interface constants, an operation not possible with define().

Usage Recommendations

Unless expressions or conditional definition is necessary, const is the preferred option due to its readability, tool compatibility, and versatility. However, define() remains useful when dynamic or conditional constant definition is required.

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