docs: add docblock to php() method in PhpProject

[Copilot]

Addresses missing documentation for the php() method that implements non-trivial version detection logic.

Changes

• Added docblock to php() method explaining:
  • Platform PHP version detection via composer show --platform php
  • Version caching using static property
  • Parameters and return value

/**
 * Create a PHP container with the appropriate PHP version.
 *
 * This method detects the platform PHP version using Composer and creates a
 * container using the appropriate PHP image. The version is cached in a static
 * property to avoid repeated lookups.
 *
 * @param Directory $source The source directory containing composer.json
 * @param string $image The base PHP image name (default: "php")
 * @param string $variant The PHP image variant (default: "cli")
 * @return Container A container with the appropriate PHP version installed
 */
private function php(
    Directory $source,
    string $image = "php",
    string $variant = "cli",
): Container {

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