Within the Intelephense plugin there is a setting to exclude files from language server features

@ext:bmewburn.vscode-intelephense-client exc

Within these exclusions I found the **/vendor/** folder. This plugin and its settings were shared with me by the previous developer and as such I was unaware of this.

The basic premise being the Intelephense was unaware of all of the symfony classes as it was unable to index that location.

Whilst looking to resolve this I have seen multiple people having similar issues with other PHP Frameworks that do not have an answer, so I will mention this post anywhere else I see that problem in the hope it will fix those too (CakePHP and Laravel being the ones I have seen)