Webmozart\Assert\Assert::string PHP Method

    public static function string($value, $message = '')
    {
        if (!is_string($value)) {
            static::reportInvalidArgument(sprintf($message ?: 'Expected a string. Got: %s', static::typeToString($value)));
        }
    }

 /**
  * Adds a participation to the data collector.
  *
  * @param string $testIdentifier It will look like "Qp0gahJ3RAO3DJ18b0XoUQ"
  * @param int $variationIndex
  * @throws InvalidArgumentException
  */
 public function addParticipation($testIdentifier, $variationIndex)
 {
     Assert::string($testIdentifier, 'Test identifier must be a string');
     Assert::integer($variationIndex, 'Variation index must be integer');
     Assert::greaterThan($variationIndex, -1, 'Variation index must be integer >= 0');
     $this->participations[$testIdentifier] = $variationIndex;
 }