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I've been building Wordpress widgets for a while and have always used some code like this:

$instance = wp_parse_args( (array) $instance);

It's never caused problems and is recommended in several places (by Justin Tadlock, two Wordpress books I have, etc.), but none of these sources really explain why.

So, what does this actually do, and what would happen if it was omitted?

Answers

In layman's terms it's merging arrays.

It's used frequently because it allows a function to accept multiple arguments without making the code look messy. Then goes the next step by allowing the developer to set up default values.

That's where wp_parse_args comes in, it merges passed in values with defaults.

$args = wp_parse_args($passed_in_args, $default_values);

It also converts a URL query string into an array.

Hope this helps