Is plugin installed

is-plugin-installed.php

$installed_plugins = array();
// check if ACF is installed
if ( is_multisite() ) :

	$network_plugins = get_site_option( 'active_sitewide_plugins', array() );
	$site_plugins = get_option( 'active_plugins', array() );
	$installed_plugins = array_merge($network_plugins, $site_plugins);

else :

	$installed_plugins = get_option( 'active_plugins', array() );

endif;

foreach($installed_plugins as $key => $value ){
	if ( strpos($key, 'acf.php') !== false || strpos($value, 'acf.php') !== false ){
		$acf_active = true;
		return;
	}
}