Is there any way to override a WordPress template with a plugin?

I'd like to make a landing page. If plugin detects some GET or POST requests it should override wordpress theme and show its own.

It would work somehow like that:

if (isset($_GET['action']) && $_GET['action'] == 'myPluginAction'){
    /* do something to maintain action */
    /* forbid template to display and show plugin's landing page*/
}

I'm familiar with WP Codex, but I don't remember if there is any function to do that. Of course, I googled it with no results.

Thanks for any ideas in advance.

Answers

You need the hook template_include. It doesn't seem documented in the Codex, but you can find more examples here in SO or in WordPress StackExchange

Plugin file

<?php
/**
 * Plugin Name: Landing Page Custom Template
 */
add_filter( 'template_include', 'so_13997743_custom_template' );

function so_13997743_custom_template( $template )
{
    if( isset( $_GET['mod']) && 'yes' == $_GET['mod'] )
        $template = plugin_dir_path( __FILE__ ) . 'my-custom-page.php';

    return $template;
}

Custom Template in Plugin folder

<?php
/**
 * Custom Plugin Template
 * File: my-custom-page.php
 *
 */

echo get_bloginfo('name');

Result

Visiting any url of the site with ?mod=yes will render the plugin template file, e.g.: http://example.com/hello-world/?mod=yes.

<?php
/**
 * Plugin Name: Custom Plugin
 */

function myplugin_plugin_path() {   
  // gets the absolute path to this plugin directory 
  return untrailingslashit( plugin_dir_path( __FILE__ ) ); 
}

add_filter( 'woocommerce_locate_template', 'myplugin_woocommerce_locate_template', 10, 3 ); 
function myplugin_woocommerce_locate_template( $template, $template_name, $template_path ) {

  global $woocommerce;  
  $_template = $template; 
  if ( ! $template_path ) $template_path = $woocommerce->template_url; 
  $plugin_path  = myplugin_plugin_path() . '/woocommerce/'; 
  // Look within passed path within the theme - this is priority 
  $template = locate_template( 
    array( 
      $template_path . $template_name, $template_name 
    ) 
  );

  // Modification: Get the template from this plugin, if it exists 
  if ( ! $template && file_exists( $plugin_path . $template_name ) ) 
    $template = $plugin_path . $template_name;  

  // Use default template 
  if ( ! $template ) 
    $template = $_template; 

  // Return what we found 
  return $template; 
 }
?>

for reference template override using plugin

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