26     protected $configuration = [  variable in Elastica\\ClientConfiguration
118             return $this->configuration;
125         return $this->configuration[$key];
133         return \array_key_exists($key, $this->configuration);
141         return $this->configuration;
150         $this->configuration[$key] = $value;
161         if (!\array_key_exists($key, $this->configuration)) {
162             $this->configuration[$key] = [$value];
164             if (\is_array($this->configuration[$key])) {
165                 $this->configuration[$key][] = $value;
167                 $this->configuration[$key] = [$this->configuration[$key], $value];