As I understand from your question you haven't added the external PHP IntelliSense extension and using the default PHP IntelliSense provided by the VS Code, If that's the situation you need to add the extension to your Visual Studio Code Editor.
To add the extension use keyboard and press Ctrl+Shift+p you will get the command palette and type the following command Extensions: Install Extension
then on to the left panel of the editor you will get the option to search for extensions, you may search for PHP IntelliSense
and install it. Editor will ask to restart the editor to apply the extension.
I recommend to disable VS Code's built-in PHP IntelliSense by adding the following property "php.suggest.basic":false
to the settings json config of the editor (keyboard press Ctrl+, you will get the settings json to the right of the editor) to avoid duplicate suggestions.
For PHP IntelliSense to work You need to have atleast PHP 7 installed, You can either add it to your PATH or set the "php.executablePath":"php physical path"
in the settings json (keyboard press Ctrl+, you will get the settings json)