Replace single_template filter with what for default posts?

Question

The answers you want are in the code. Meaning, you need to go exploring.

Your search should start with wp-includes/template-loader.php. This is the very last file WordPress includes before the theme get’s loaded. it runs through a bunch of conditionals to check which type of page we’re out and what template to fetch.

The relevant (offending?) line:

<?php
elseif ( is_single() && $template = get_single_template() ) :

Now that we know get_single_template fetches templates for single posts, we can go find that function in wp-includes/template.php.

<?php
/**
 * Retrieve path of single template in current or parent template.
 *
 * @since 1.5.0
 *
 * @return string
 */
function get_single_template() {
    $object = get_queried_object();

    $templates = array();

    $templates[] = "single-{$object->post_type}.php";
    $templates[] = "single.php";

    return get_query_template( 'single', $templates );
}

No filters yet, but you’re starting to see how WP locates templates. Now we need to see what get_query_template does (get_query_template is also in wp-includes/template.php).

<?php
/**
 * Retrieve path to a template
 *
 * Used to quickly retrieve the path of a template without including the file
 * extension. It will also check the parent theme, if the file exists, with
 * the use of {@link locate_template()}. Allows for more generic template location
 * without the use of the other get_*_template() functions.
 *
 * @since 1.5.0
 *
 * @param string $type Filename without extension.
 * @param array $templates An optional list of template candidates
 * @return string Full path to file.
 */
function get_query_template( $type, $templates = array() ) {
    $type = preg_replace( '|[^a-z0-9-]+|', '', $type );

    if ( empty( $templates ) )
        $templates = array("{$type}.php");

    return apply_filters( "{$type}_template", locate_template( $templates ) );
}

The first argument is the type of template we’re goign to be locating (single for this question) and the second is a series of templates to search. This is the point where we stop. locate_template just searches the child and parent theme directories for each of the files in the $templates array — it returns the file path if one is found or an empty string if it isn’t.

Notice that $type in this function is “single”.

I wrote all the above, so I could tell you that the filter is still single_template. The actual string single_template is not found in the source, but the filter still exists (and it works).

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