WordPress offers a lot of hooks you can use to interact with code. Filters are used to change a value of a passed variable, and you usually want to change this value every time this filter is applied. But for actions you might want to run them only once, especially when thy run side effects like sending an email.

Check, if an action has been triggered

With the function did_action() you can check how many times an action has already been called. If you want to run your custom code only when the action is run the first time and then not a second time, you can do the following:

function do_this_only_once() {
	// If the action we are hooking in was called more than once, return.
	if ( did_action( 'the_hook_name' ) > 1 ) {
		return;
	}

	// Run your custom code
}
add_action( 'the_hook_name', 'do_this_only_once' );

When the the_hook_name action is being called and then running did_action( 'the_hook_name' ), the return value will be 1, as the action has just been triggered. Therefore, you can’t simply use its return value as a boolean, but you have to check if the return value is greater than one, to stop the execution of your custom code.

Use your own action to avoid multiple runs of your code

Sometimes you cannot simply break check if the action runs for the first time, but you have to check for additional things. You can add them all to the one condition. Alternatively, you can use your own action, which then you use in the early return condition:

function do_this_only_once( $hook_parameter ) {
	// If the custom code has been run already, return.
	if ( did_action( 'do_this_only_once' ) ) {
		return;
	}

	// A second check on a hook parameter.
	if ( 'something' !== $hook_parameter ) {
		return;
	}

	// Run your custom code

	// Call our custom action, so we can check, if it has been called already.
	do_action( 'do_this_only_once' );
}
add_action( 'the_hook_name', 'do_this_only_once' );

In this code example, we use a custom action in our first condition. Then we perform some other checks, to prevent execution of our custom code, unless we really want to run it. Then we finally run our code. At the very end of the function, we run our custom action, so we can use it on the next call to prevent a second execution. This early return makes sure we not only run the code once, but also that we don’t run the additional conditions again.

Conclusion

There are different ways to make sure to run an action only once. It’s usually best to just use the did_action() function which will tell you how many times an action was run. And if you have some side effects in your code, like sending an email, you really should make sure you don’t run them more times than necessary.