Where is the key _wp_page_template passed?

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When adding custom post meta boxes, I understand that whatever name attribute input you put would be passed through the form into the server.

Hence, If I have

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<input type="text" name="my_custom_field" id="my_custom_field" />

using 

get_post_meta($post->ID, 'my_custom_field', true);

I am able to retrieve the value of that input is possible.

Regarding this, I did some googling on how to target custom page templates and this was what I found. 

global $post;
function func_name {
    if ( 'foobar.php' == get_post_meta( $post->ID, '_wp_page_template', true ) ) {
         do something...
     }
}
add_action( 'add_meta_boxes_page', 'func_name' );

Where does _wp_page_template come from?

I did a inspect on the WP backend edit page on the Page Attributes meta box but no fields have that key name on it. What boggles me is how _wp_page_template also appears in the database wp_postmeta table.

Can anyone help me explain this?

Thanks in advance.

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2 comments

  1. The HTML attribute name is what is sent to the server. There is some PHP code reading it and storing it with a specific key in the database.

    The HTML name and the database key don’t have to be the same string. The PHP code can use a completely different name or split one value into multiple database values.

    The template attribute handler does that: It takes the name page_template from page attributes metabox and stores it as _wp_page_template in the database. The leading underscore protects that field from showing up in the custom fields metabox.

    See this part in wp_insert_post():

    if ( !empty($page_template) && 'page' == $data['post_type'] ) {
    $post->page_template = $page_template;
    $page_templates = wp_get_theme()->get_page_templates();
    if ( 'default' != $page_template && ! isset( $page_templates[ $page_template ] ) ) {
        if ( $wp_error )
            return new WP_Error('invalid_page_template', __('The page template is invalid.'));
        else
            return 0;
    }
    update_post_meta($post_ID, '_wp_page_template',  $page_template);
}

  2. Saving data from a form is not automatic as you seem to think…

    When adding custom post meta boxes, I understand that whatever name
    attribute input you put would be passed through the form into the
    server. Hence, if <input type="text" name="my_custom_field"
    id="my_custom_field" />    , using get_post_meta($post->ID,
    'my_custom_field', true);     to retrieve the value of that input is
    possible.

    Yes, the form data will be passed to the server as either POST or GET data but nothing happens to that data if it is not intentionally processed by the script. That often means saving to the database but not necessarily. You can email the data instead, as part of a contact form for example.

    The name attribute the form uses does not have to match the key saved to the database. The code that processes the form can save it it under any key the programmer wants.

    The particular piece of data you are talking about is processed by the wp_insert_post function in wp-includes/post.php.

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