libraries/class-recursive-arrayaccess.php
public function offsetSet( $offset, $data ) {
		if ( is_array( $data ) ) {
			$data = new self( $data );
		}
		if ( $offset === null ) { // don't forget this!
			$this->container[] = $data;
		} else {
			$this->container[ $offset ] = $data;
		}
		$this->dirty = true;
	}
public function offsetSet( $offset, $data ) {
		if ( is_array( $data ) ) {
			$data = new self( $data );
		}
		if ( $offset === null ) { // don't forget this!
			$this->container[] = $data;
		} else {
			$this->container[ $offset ] = $data;
		}
		$this->dirty = true;
	}
Offset to set

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

@param mixed $offset The offset to assign the value to.
@param mixed $value  The value to set.

@return void