src/Utils/MixedStore.php
public function offsetExists($offset)
    {
        if ($offset === false) {
            return $this->falseValueIsSet;
        }
        if ($offset === true) {
            return $this->trueValueIsSet;
        }
        if (is_int($offset) || is_string($offset)) {
            return array_key_exists($offset, $this->standardStore);
        }
        if (is_float($offset)) {
            return array_key_exists((string) $offset, $this->floatStore);
        }
        if (is_object($offset)) {
            return $this->objectStore->offsetExists($offset);
        }
        if (is_array($offset)) {
            foreach ($this->arrayKeys as $index => $entry) {
                if ($entry === $offset) {
                    $this->lastArrayKey   = $offset;
                    $this->lastArrayValue = $this->arrayValues[$index];

                    return true;
                }
            }
        }
        if ($offset === null) {
            return $this->nullValueIsSet;
        }

        return false;
    }
public function offsetExists($offset)
    {
        if ($offset === false) {
            return $this->falseValueIsSet;
        }
        if ($offset === true) {
            return $this->trueValueIsSet;
        }
        if (is_int($offset) || is_string($offset)) {
            return array_key_exists($offset, $this->standardStore);
        }
        if (is_float($offset)) {
            return array_key_exists((string) $offset, $this->floatStore);
        }
        if (is_object($offset)) {
            return $this->objectStore->offsetExists($offset);
        }
        if (is_array($offset)) {
            foreach ($this->arrayKeys as $index => $entry) {
                if ($entry === $offset) {
                    $this->lastArrayKey   = $offset;
                    $this->lastArrayValue = $this->arrayValues[$index];

                    return true;
                }
            }
        }
        if ($offset === null) {
            return $this->nullValueIsSet;
        }

        return false;
    }
Whether a offset exists

@link http://php.net/manual/en/arrayaccess.offsetexists.php

@param mixed $offset <p>
An offset to check for.
</p>

@return bool true on success or false on failure.
</p>
<p>
The return value will be casted to boolean if non-boolean was returned.