focus() event of jQuery UI autocomplete lets you to show all options before start entering. Means the autocomplete list would be shown on focusing the input box. Once start entering in the input field, autocomplete list would be filtered as per the entered character.
At the time of using jQuery UI Autocomplete in PHP, many times you need to pass additional parameters to an autoComplete call. Here is a simple way to pass extra parameters to the source URL in jQuery UI Autocomplete.