I'm having a hard time understanding how to set up a function with a default parameter and then call it with the do_action in WordPress. I'm not sure if this is a WordPress issue or just me not understanding the do_action method. What happens is I define a function like this:

function contact_info( $show_logo=true ) {

    var_dump( $show_logo );    

Then I add the hook like this:

add_action( 'show_contact_info', 'contact_info' );

Finally when I call the do_action like this:


I would expect a the var_dump to be bool(true), but what I am getting is: string(0) ""

So my question is WHY?

It doesn't seem to be accepting the default as set in my function, or more specifically, the do_action call ALWAYS seems to pass blank as the first parameter if one is not defined in the call. Is this standard for Wordpress? Why would it not pass NULL?

  • In addition to the answer birgire's answer, you can use add_action( 'show_contact_info', 'contact_info', 10, 0 ); to prevent this.
    – swissspidy
    Aug 2, 2017 at 11:33

1 Answer 1


When we fire up:

do_action( 'foo' );

we are actually calling:

do_action( 'foo', '' );

That's because how it's defined; With the empty string as the default input argument:

function do_action($tag, $arg = '') {
    // ...

So the action's callback get's the empty string as it's first input argument.

This has been like this since at least WordPress version 1.5.

Since WordPress 4.6, do_action() is a wrapper of WP_Hook::do_action();

What if ...

It looks like if it would be defined without the argument part as:

function do_action($tag) {
    // ...

and with corresponding modifications to this part:

$args = array();
if ( is_array($arg) && 1 == count($arg) && isset($arg[0]) && is_object($arg[0]) ) // array(&$this)
    $args[] =& $arg[0];
    $args[] = $arg;

then we might use the default value, of the first input argument, in our callback.

  • 1
    Check out core.trac.wordpress.org/ticket/14881 for some more information on this.
    – swissspidy
    Aug 2, 2017 at 11:31
  • thanks for the trac link @swissspidy Hopefully this will be resolved some day.
    – birgire
    Aug 2, 2017 at 11:35
  • I was just wondering about a possible patch for this, when writing the answer, but it looks like @nacin has already patched it 7 years ago here in #14881.diff ;-)
    – birgire
    Aug 2, 2017 at 11:46

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