WordPress: Check if Plugin is installed

Question

I want to check if Yoast SEO is installed in WordPress. I have activated Yoast SEO in my Test Environment, but it's not working out.

In the wp-seo-main.php of Yoast, there's this line on line 16:

define( 'WPSEO_VERSION', '3.4' );

So I thought, that's a good line to check if Yoast is installed and running, so I did:

if ( defined( 'WPSEO_VERSION' ) ) {
    echo '<script>alert("Yes, defined");</script>';
} else {
    echo '<script>alert("No, undefined");</script>';
}

But it gives me "No, undefined". How weird, because it should be defined.

Anyone got an idea? I'm totally out of ideas.

Answer 1

Thanks to all the others for doing a good job so far! But the question is also about a check if a plugin is installed and all your answers are only about checking if a plugin is active.

So I went to my playground and developed a correct check I want to show you by using functions WP already provided.

// Check if needed functions exists - if not, require them
if ( ! function_exists( 'get_plugins' ) || ! function_exists( 'is_plugin_active' ) ) {
    require_once ABSPATH . 'wp-admin/includes/plugin.php';
}

/**
 * Checks if Yoast SEO is installed
 *
 * @return bool
 */
function is_wp_seo_installed(): bool {
    if ( check_plugin_installed( 'wordpress-seo/wp-seo.php' ) ) {
        return true;
    }

    return false;
}

/**
 * Check if Yoast SEO is activated
 *
 * @return bool
 */
function is_wp_seo_active(): bool {
    if ( check_plugin_active( 'wordpress-seo/wp-seo.php' ) ) {
        return true;
    }

    return false;
}

/**
 * Check if plugin is installed by getting all plugins from the plugins dir
 *
 * @param $plugin_slug
 *
 * @return bool
 */
function check_plugin_installed( $plugin_slug ): bool {
    $installed_plugins = get_plugins();

    return array_key_exists( $plugin_slug, $installed_plugins ) || in_array( $plugin_slug, $installed_plugins, true );
}

/**
 * Check if plugin is installed
 *
 * @param string $plugin_slug
 *
 * @return bool
 */
function check_plugin_active( $plugin_slug ): bool {
    if ( is_plugin_active( $plugin_slug ) ) {
        return true;
    }

    return false;
}

As you can see I've wrote two separate functions to check if Yoast SEO is installed and active so I just need to call it and check the return param:

$installed = is_wp_seo_installed();
$active    = is_wp_seo_active();

if ( $installed && $active ) {
    echo 'WP SEO is installed and active!';
} else {
    echo 'WP SEO is not installed or active!';
}

But if you want to skip this two extra functions, you can call the two check functions directly:

$installed = check_plugin_installed( 'wordpress-seo/wp-seo.php' );
$active    = check_plugin_active( 'wordpress-seo/wp-seo.php' );

if ( $installed && $active ) {
    echo 'Yoast SEO is installed and active!';
} else {
    echo 'Yoast SEO is not installed or active!';
}

I hope this helps you to do a good installed & active check!

Answer 2

Or without extra inclusions, front end or back end:

if ( in_array( 'wordpress-seo/wp-seo.php', apply_filters( 'active_plugins', get_option( 'active_plugins' ) ) ) ) {
    // do stuff only if Yoast is installed and active
}

Works with any plugin, just look into your plugins folder for your target: plugin-folder/plugin-index-name.php where the latter shuld resident the Plugin details at the top inside the file.

Please Check Doc. reference

Answer 3

Use below code:

 <?php include_once ABSPATH . 'wp-admin/includes/plugin.php'; ?>
 <?php if ( is_plugin_active( 'wordpress-seo/wp-seo.php' ) ) :

      //do staff

 <?php endif; ?>