Your edit got me to this idea, there indeed is no function called »woocommerce«, there is a class »WooCommerce« though. One thing to be aware of is, that the check has to late enough, so that plug-ins are actually initialized, otherwise – obviously – the class won’t exists and the check returns false. So your check should look like this:
if ( class_exists( 'WooCommerce' ) ) {
// some code
} else {
/ more code
}
On the WooCommerce documentation page »Creating a plugin for WooCommerce« I have found this:
/**
* Check if WooCommerce is active
**/
if (
in_array(
'woocommerce/woocommerce.php',
apply_filters( 'active_plugins', get_option( 'active_plugins' ) )
)
) {
// Put your plugin code here
}
Personally I think it is not nearly as reliable as checking for the class. For obvious reasons, what if the folder/file name is different, but should work just fine too. Besides if you do it like this, then there is an API function you can use, fittingly named is_plugin_active(). But because it is normally only available on admin pages, you have to make it available by including wp-admin/includes/plugin.php, where the function resides. Using it the check would look like this:
include_once( ABSPATH . 'wp-admin/includes/plugin.php' );
if ( is_plugin_active( 'woocommerce/woocommerce.php' ) ) {
// some code
} else {
/ more code
}