PSR-4 maps namespace prefixes to directories and expects file and class case to match across operating systems.
What It Does
The namespace prefix App\ maps to the src directory; separators become path separators. Namespace, class name and case-sensitive path must match the Composer mapping.
Requirements and Compatibility
Use a supported PHP runtime and install any third-party tool or extension explicitly shown in the examples. Commands should be run from the project root unless stated otherwise.
Syntax, Inputs and Results
The examples separate configuration, the smallest working operation and a realistic follow-up task. Adapt file paths and dependency versions to your own project.
Example 1: Basic Use
The namespace prefix App\ maps to the src directory; separators become path separators.
{
"autoload": {"psr-4": {"Warehouse\\": "src/"}}
}Example 2: Another Case
Namespace, class name and case-sensitive path must match the Composer mapping.
<?php
// File: src/Domain/Invoice.php
namespace Warehouse\Domain;
final class Invoice { public function total(): int { return 250; } }Practical Example
Loading vendor/autoload.php registers the Composer loader before class use.
<?php
require __DIR__ . '/vendor/autoload.php';
$invoice = new Warehouse\Domain\Invoice();
echo $invoice->total();For production, apply the compatibility and edge-case limits described below.
Important Behavior and Edge Cases
Case mismatches that work locally on Windows may fail on Linux.
Comparison with Related Functions and Tools
| Subject | Key difference |
|---|---|
| Composer autoloading vs manual require | Composer maps namespaces to source files; require loads one file explicitly at the call site. |
Common Beginner Mistakes
| Mistake | Correct approach |
|---|---|
| Mismatching namespace and directory names | Make case and path separators match the PSR-4 map, especially on Linux. |
| Skipping the prerequisite of this specific workflow | Install or configure the named tool and confirm the example can actually run. |
| Ignoring edge conditions | Case mismatches that work locally on Windows may fail on Linux. |
Key Takeaways
- PSR-4 maps namespace prefixes to directories and expects file and class case to match across operating systems.
- Case mismatches that work locally on Windows may fail on Linux.
- Keep application configuration, source code and validation tests under version control.
Use these examples as a starting point, then validate the result with the actual PHP build, dependencies and inputs used by your application.