What does it mean when a PHP function name begins with an underscore?
for example: __construct()
I know what the construct means but I've seen other places where the function begins with an underscore, or a double underscore and I'm just not sure of the significance.
It means that PHP calls it implicitly.
It's called a 'Magic Method'
Also, it's two underscores, not one.
Learn more here: PHP Magic Methods
In PHP, functions start with two underscores usually have special meanings. From the manual:
PHP reserves all function names starting with __ as magical. It is recommended that you do not use function names with __ in PHP unless you want some documented magic functionality.
For example __construct() is a special method which is called automatically while initializing an object.
As noted in other answers, the double underscore is used for "magic" methods. The idea is that the user would never intentionally use two underscores for a method name, so there is little risk of collision. The reason it isn't a single underscore, I believe, is that the single underscore was a popular convention for private methods in the bad(der) old days, before the 'private' keyword came along to enforce OO privateness.
In codeigniter, if you start a method in a controller with an underscore, the router will know not to allow it to be accessed from the url.
