PHP: Class constants in type declarations of input paramaters

Question

For a custom logger I want to force the caller to pass a valid class constant defined in Psr\Log\LogLevel.

This class is defined like:

namespace Psr\Log;
/**
 * Describes log levels.
 */
class LogLevel
{
    const EMERGENCY = 'emergency';
    const ALERT     = 'alert';
    const CRITICAL  = 'critical';
    const ERROR     = 'error';
    const WARNING   = 'warning';
    const NOTICE    = 'notice';
    const INFO      = 'info';
    const DEBUG     = 'debug';
}

The loggers' function (wrong) looks like:

public static function log($log, LogLevel $logLevel = null): void {
   // .....
}

This does not work because LogLevel::DEBUG for instance is a string and not an instance of the class. Is there any way to enforce a class constant in a PHP type declaration? Because if I define string then you can pass any string obviously, but I just want to allow the declared constants.

Answer 1

PHP doesn't have constant restrictions, only types.

But you can do a workaround like this:

class LogLevel
{
    protected string $logName;

    private function __construct(string $logName)
    {
        $this->logName = $logName;
    }
    public function getName(): string
    {
        return $this->logName;
    }

    public static function emergency(): self
    {
        return new self('emergency');
    }

    public static function alert(): self
    {
        return new self('alert');
    }

    public static function critical(): self
    {
        return new self('critical');
    }
    // create the other constants here
}

Now your static funcion works

public static function log($log, LogLevel $logLevel = null): void {
   // .....
}

$logLevel will receive LogLevel::emergency(), LogLevel::critical(), etc. and you can get the level name just calling $logLevel->getName()

Answer 2

If you have Phpstorm you could use new #[ExpectedValues] attribute feature: https://blog.jetbrains.com/phpstorm/2020/10/phpstorm-2020-3-eap-4/#expectedvalues_examples

Answer 3

There is not an easy way but you could check if the inserted string matches certain values

function checkLog(string $logLevel){
  $acceptableValues = array("emergency", "error", "debug");
  if(!in_array($logLevel, $acceptableValues) die();
}

Replace the strings in the array with the consts of your class.