How to distinguish if a plugin is not installed or just not active

file_exists expects a path, not a URL. To get the path to an arbitrary plugin you’ll need to use WP_PLUGIN_DIR:

$pathpluginurl = WP_PLUGIN_DIR . '/advanced-custom-fields/acf.php';

$isinstalled = file_exists( $pathpluginurl );

Hata!: SQLSTATE[HY000] [1045] Access denied for user 'divattrend_liink'@'localhost' (using password: YES)