Set up processing instruction (PI) handler

# xml_set_processing_instruction_handler

(PHP 4, PHP 5, PHP 7, PHP 8)

xml_set_processing_instruction_handler — Set up processing instruction (PI) handler

### Description

xml_set_processing_instruction_handler(XMLParser `$parser`, callable|string|null `$handler`): true

Sets the processing instruction (PI) handler function for the XML parser `parser`.

A processing instruction has the following format:

    <?target
    data
    ?>

Caution

PHP code is delimited by the `<?php` processing instruction. As such it is possible to have PHP code within an XML document. However, the PI end tag (`?>`) must not be part of the data. If a PI end tag exists as part of the embedded PHP code, the rest of the PHP code and the "real" PI end tag, will be treated as character data.

### Parameters

`parser`

  The XML parser.

`handler`

  If `null` is passed, the handler is reset to its default state.

  Warning

  An empty string will also reset the handler, however this is deprecated as of PHP 8.4.0.

  If `handler` is a callable, the callable is set as the handler.

  If `handler` is a string, it can be the name of a method of an object set with xml_set_object().

  Warning

  This is deprecated as of PHP 8.4.0.

  Warning

  As of PHP 8.4.0, the callable is checked to be valid while setting the handler, not when it is called. This means that xml_set_object() must be called prior to setting a method string as the callback. However, as this behaviour is also deprecated as of PHP 8.4.0, using a proper callable for the method is recommended instead.

  The signature of the handler must be:

  handler(XMLParser `$parser`, string `$target`, string `$data`): void
  `parser`
    The XML parser calling the handler.
  `target`
    The processing instruction target.
  `data`
    The processing instruction data.

### Return Values

Always returns `true`.

### Changelog

Version | Description
--- | ---
8.4.0 | Passing a non-callable string to `handler` is now deprecated, use a proper callable for methods, or `null` to reset the handler.
8.4.0 | The validity of `handler` as a callable is now checked when setting the handler instead of checking when calling it.
8.0.0 | `parser` expects an XMLParser instance now; previously, a valid `xml` resource was expected.

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