I saw this piece of code that have part of the code I never saw before. What is the behavior of (array) in this scenario? so far I know the way of creating arrays in php never see this.
//
$this->settings = array_merge( $this->settings_defaults(), (array) get_option( $this->settings_slug, $this->settings_defaults() ) );
//
You are looking at type casting: http://php.net/manual/en/language.types.type-juggling.php
What the code does is caste the value returned by get_option() to an array. It is being done so that array_merge() works correctly and doesn't trigger warnings/errors. That much is pure PHP and is off-topic. The only reason I chose to answer rather than to post a comment is because the return value of get_option() will return various types of data depending on context. You can see that noted in the source:
@param bool|mixed $pre_option Value to return instead of the option value.
https://core.trac.wordpress.org/browser/tags/4.0/src/wp-includes/option.php#L42
That means that you have to manipulate the return data in order to be sure of the type of data that you are dealing with.
