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/includes/ -> functions.plugin-api.php (summary)

The plugin API is located in this file, which allows for creating actions and filters and hooking functions, and methods. The functions or methods will then be run when the action or filter is called. The API callback examples reference functions, but can be methods of classes. To hook methods, you'll need to pass an array one of two ways.

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Defines 22 functions

  add_filter()
  has_filter()
  apply_filters()
  apply_filters_ref_array()
  remove_filter()
  remove_all_filters()
  current_filter()
  add_action()
  do_action()
  did_action()
  do_action_ref_array()
  has_action()
  remove_action()
  remove_all_actions()
  plugin_basename()
  plugin_dir_path()
  plugin_dir_url()
  register_activation_hook()
  register_deactivation_hook()
  register_uninstall_hook()
  _wp_call_all_hook()
  _wp_filter_build_unique_id()

Functions
Functions that are not part of a class:

add_filter($tag, $function_to_add, $priority = 10, $accepted_args = 1)   X-Ref
Hooks a function or method to a specific filter action.

Filters are the hooks that WordPress launches to modify text of various types
before adding it to the database or sending it to the browser screen. Plugins
can specify that one or more of its PHP functions is executed to
modify specific types of text at these times, using the Filter API.

To use the API, the following code should be used to bind a callback to the
filter.

<code>
function example_hook($example) { echo $example; }
add_filter('example_filter', 'example_hook');
</code>

In WordPress 1.5.1+, hooked functions can take extra arguments that are set
when the matching do_action() or apply_filters() call is run. The
$accepted_args allow for calling functions only when the number of args
match. Hooked functions can take extra arguments that are set when the
matching do_action() or apply_filters() call is run. For example, the action
comment_id_not_found will pass any functions that hook onto it the ID of the
requested comment.

<strong>Note:</strong> the function will return true no matter if the
function was hooked fails or not. There are no checks for whether the
function exists beforehand and no checks to whether the <tt>$function_to_add
is even a string. It is up to you to take care and this is done for
optimization purposes, so everything is as quick as possible.

since: 0.71
param: string $tag The name of the filter to hook the $function_to_add to.
param: callback $function_to_add The name of the function to be called when the filter is applied.
param: int $priority optional. Used to specify the order in which the functions associated with a particular action are executed (default: 10). Lower numbers correspond with earlier execution, and functions with the same priority are executed in the order in which they were added to the action.
param: int $accepted_args optional. The number of arguments the function accept (default 1).
return: boolean true

has_filter($tag, $function_to_check = false)   X-Ref
Check if any filter has been registered for a hook.

since: 2.5
param: string $tag The name of the filter hook.
param: callback $function_to_check optional.  If specified, return the priority of that function on this hook or false if not attached.
return: int|boolean Optionally returns the priority on that hook for the specified function.

apply_filters($tag, $value)   X-Ref
Call the functions added to a filter hook.

The callback functions attached to filter hook $tag are invoked by calling
this function. This function can be used to create a new filter hook by
simply calling this function with the name of the new hook specified using
the $tag parameter.

The function allows for additional arguments to be added and passed to hooks.
<code>
function example_hook($string, $arg1, $arg2)
{
//Do stuff
return $string;
}
$value = apply_filters('example_filter', 'filter me', 'arg1', 'arg2');
</code>

since: 0.71
param: string $tag The name of the filter hook.
param: mixed $value The value on which the filters hooked to <tt>$tag</tt> are applied on.
param: mixed $var,... Additional variables passed to the functions hooked to <tt>$tag</tt>.
return: mixed The filtered value after all hooked functions are applied to it.

apply_filters_ref_array($tag, $args)   X-Ref
Execute functions hooked on a specific filter hook, specifying arguments in an array.

since: 3.0.0
param: string $tag The name of the filter hook.
param: array $args The arguments supplied to the functions hooked to <tt>$tag</tt>
return: mixed The filtered value after all hooked functions are applied to it.

remove_filter($tag, $function_to_remove, $priority = 10, $accepted_args = 1)   X-Ref
Removes a function from a specified filter hook.

This function removes a function attached to a specified filter hook. This
method can be used to remove default functions attached to a specific filter
hook and possibly replace them with a substitute.

To remove a hook, the $function_to_remove and $priority arguments must match
when the hook was added. This goes for both filters and actions. No warning
will be given on removal failure.

since: 1.2
param: string $tag The filter hook to which the function to be removed is hooked.
param: callback $function_to_remove The name of the function which should be removed.
param: int $priority optional. The priority of the function (default: 10).
param: int $accepted_args optional. The number of arguments the function accpets (default: 1).
return: boolean Whether the function existed before it was removed.

remove_all_filters($tag, $priority = false)   X-Ref
Remove all of the hooks from a filter.

since: 2.7
param: string $tag The filter to remove hooks from.
param: int $priority The priority number to remove.
return: bool True when finished.

current_filter()   X-Ref
Retrieve the name of the current filter or action.

since: 2.5
return: string Hook name of the current filter or action.

add_action($tag, $function_to_add, $priority = 10, $accepted_args = 1)   X-Ref
Hooks a function on to a specific action.

Actions are the hooks that the WordPress core launches at specific points
during execution, or when specific events occur. Plugins can specify that
one or more of its PHP functions are executed at these points, using the
Action API.

since: 1.2
param: string $tag The name of the action to which the $function_to_add is hooked.
param: callback $function_to_add The name of the function you wish to be called.
param: int $priority optional. Used to specify the order in which the functions associated with a particular action are executed (default: 10). Lower numbers correspond with earlier execution, and functions with the same priority are executed in the order in which they were added to the action.
param: int $accepted_args optional. The number of arguments the function accept (default 1).

do_action($tag, $arg = '')   X-Ref
Execute functions hooked on a specific action hook.

This function invokes all functions attached to action hook $tag. It is
possible to create new action hooks by simply calling this function,
specifying the name of the new hook using the <tt>$tag</tt> parameter.

You can pass extra arguments to the hooks, much like you can with
apply_filters().

since: 1.2
param: string $tag The name of the action to be executed.
param: mixed $arg,... Optional additional arguments which are passed on to the functions hooked to the action.
return: null Will return null if $tag does not exist in $wp_filter array

did_action($tag)   X-Ref
Retrieve the number times an action is fired.

since: 2.1
param: string $tag The name of the action hook.
return: int The number of times action hook <tt>$tag</tt> is fired

do_action_ref_array($tag, $args)   X-Ref
Execute functions hooked on a specific action hook, specifying arguments in an array.

since: 2.1
param: string $tag The name of the action to be executed.
param: array $args The arguments supplied to the functions hooked to <tt>$tag</tt>
return: null Will return null if $tag does not exist in $wp_filter array

has_action($tag, $function_to_check = false)   X-Ref
Check if any action has been registered for a hook.

since: 2.5
param: string $tag The name of the action hook.
param: callback $function_to_check optional.  If specified, return the priority of that function on this hook or false if not attached.
return: int|boolean Optionally returns the priority on that hook for the specified function.

remove_action($tag, $function_to_remove, $priority = 10, $accepted_args = 1)   X-Ref
Removes a function from a specified action hook.

This function removes a function attached to a specified action hook. This
method can be used to remove default functions attached to a specific filter
hook and possibly replace them with a substitute.

since: 1.2
param: string $tag The action hook to which the function to be removed is hooked.
param: callback $function_to_remove The name of the function which should be removed.
param: int $priority optional The priority of the function (default: 10).
param: int $accepted_args optional. The number of arguments the function accpets (default: 1).
return: boolean Whether the function is removed.

remove_all_actions($tag, $priority = false)   X-Ref
Remove all of the hooks from an action.

since: 2.7
param: string $tag The action to remove hooks from.
param: int $priority The priority number to remove them from.
return: bool True when finished.

plugin_basename($file)   X-Ref
Gets the basename of a plugin.

This method extracts the name of a plugin from its filename.

since: 1.5
param: string $file The filename of plugin.
return: string The name of a plugin.

plugin_dir_path( $file )   X-Ref
Gets the filesystem directory path (with trailing slash) for the plugin __FILE__ passed in

since: 2.8
param: string $file The filename of the plugin (__FILE__)
return: string the filesystem path of the directory that contains the plugin

plugin_dir_url( $file )   X-Ref
Gets the URL directory path (with trailing slash) for the plugin __FILE__ passed in

since: 2.8
param: string $file The filename of the plugin (__FILE__)
return: string the URL path of the directory that contains the plugin

register_activation_hook($file, $function)   X-Ref
Set the activation hook for a plugin.

When a plugin is activated, the action 'activate_PLUGINNAME' hook is
activated. In the name of this hook, PLUGINNAME is replaced with the name of
the plugin, including the optional subdirectory. For example, when the plugin
is located in wp-content/plugin/sampleplugin/sample.php, then the name of
this hook will become 'activate_sampleplugin/sample.php'. When the plugin
consists of only one file and is (as by default) located at
wp-content/plugin/sample.php the name of this hook will be
'activate_sample.php'.

since: 2.0
param: string $file The filename of the plugin including the path.
param: callback $function the function hooked to the 'activate_PLUGIN' action.

register_deactivation_hook($file, $function)   X-Ref
Set the deactivation hook for a plugin.

When a plugin is deactivated, the action 'deactivate_PLUGINNAME' hook is
deactivated. In the name of this hook, PLUGINNAME is replaced with the name
of the plugin, including the optional subdirectory. For example, when the
plugin is located in wp-content/plugin/sampleplugin/sample.php, then
the name of this hook will become 'activate_sampleplugin/sample.php'.

When the plugin consists of only one file and is (as by default) located at
wp-content/plugin/sample.php the name of this hook will be
'activate_sample.php'.

since: 2.0
param: string $file The filename of the plugin including the path.
param: callback $function the function hooked to the 'activate_PLUGIN' action.

register_uninstall_hook($file, $callback)   X-Ref
Set the uninstallation hook for a plugin.

Registers the uninstall hook that will be called when the user clicks on the
uninstall link that calls for the plugin to uninstall itself. The link won't
be active unless the plugin hooks into the action.

The plugin should not run arbitrary code outside of functions, when
registering the uninstall hook. In order to run using the hook, the plugin
will have to be included, which means that any code laying outside of a
function will be run during the uninstall process. The plugin should not
hinder the uninstall process.

If the plugin can not be written without running code within the plugin, then
the plugin should create a file named 'uninstall.php' in the base plugin
folder. This file will be called, if it exists, during the uninstall process
bypassing the uninstall hook. The plugin, when using the 'uninstall.php'
should always check for the 'WP_UNINSTALL_PLUGIN' constant, before
executing.

since: 2.7
param: string $file
param: callback $callback The callback to run when the hook is called.

_wp_call_all_hook($args)   X-Ref
Calls the 'all' hook, which will process the functions hooked into it.

The 'all' hook passes all of the arguments or parameters that were used for
the hook, which this function was called for.

This function is used internally for apply_filters(), do_action(), and
do_action_ref_array() and is not meant to be used from outside those
functions. This function does not check for the existence of the all hook, so
it will fail unless the all hook exists prior to this function call.

since: 2.5
param: array $args The collected parameters from the hook that was called.
param: string $hook Optional. The hook name that was used to call the 'all' hook.

_wp_filter_build_unique_id($tag, $function, $priority)   X-Ref
Build Unique ID for storage and retrieval.

The old way to serialize the callback caused issues and this function is the
solution. It works by checking for objects and creating an a new property in
the class to keep track of the object and new objects of the same class that
need to be added.

It also allows for the removal of actions and filters for objects after they
change class properties. It is possible to include the property $wp_filter_id
in your class and set it to "null" or a number to bypass the workaround.
However this will prevent you from adding new classes and any new classes
will overwrite the previous hook by the same class.

Functions and static method callbacks are just returned as strings and
shouldn't have any speed penalty.

since: 2.2.3
param: string $tag Used in counting how many hooks were applied
param: callback $function Used for creating unique id
param: int|bool $priority Used in counting how many hooks were applied.  If === false and $function is an object reference, we return the unique id only if it already has one, false otherwise.
param: string $type filter or action
return: string|bool Unique ID for usage as array key or false if $priority === false and $function is an object reference, and it does not already have a uniqe id.



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