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<?php

//some file override
add_filter( 'page_template', 'wpa3396_page_template' );
function wpa3396_page_template( $page_template )
{
    if ( is_page( 'my-custom-page-slug' ) ) {
        $page_template = dirname( __FILE__ ) . '/custom-page-template.php';
    }
    return $page_template;
}

//for single post ovverirde
https://codex.wordpress.org/Plugin_API/Filter_Reference/single_template
function get_custom_post_type_template($single_template) {
     global $post;

     if ($post->post_type == 'my_post_type') {
          $single_template = dirname( __FILE__ ) . '/post-type-template.php';
     }
     return $single_template;
}
add_filter( 'single_template', 'get_custom_post_type_template' );


//All page override
https://github.com/wpexplorer/page-templater

//Template redirect
function my_page_template_redirect()
{
    if( is_page( 'goodies' ) && ! is_user_logged_in() )
    {
        wp_redirect( home_url( '/signup/' ) );
        die;
    }
}
add_action( 'template_redirect', 'my_page_template_redirect' );


//template override 2
add_filter( 'template_include', 'portfolio_page_template', 99 );
function portfolio_page_template( $template ) {

	if ( is_page( 'portfolio' ) ) {
		$new_template = locate_template( array( 'portfolio-page-template.php' ) );
		if ( !empty( $new_template ) ) {
			return $new_template;
		}
	}

	return $template;
}