This extension may conflict with other PHP extensions which provide similar functionality. Should you experience duplicate suggestions, incorrect results and/or poor performance, try disabling other extensions first before reporting an issue. Disabling the built-in VSCode PHP Language Features is recommended. Please check the configuration section for language server options.
Suggested remediation: Disable completion in phan's settings or in the other plugin's settings
e.g. for https://marketplace.visualstudio.com/items?itemName=bmewburn.vscode-intelephense-client it looks like intelliphense's maintainers have encountered similar issues - disabling the individual features (hover, completion, etc.) of either phan/intelliphense may be enough to fix this
https://github.com/tysonandre/vscode-php-phan#completion
This issue duplicates the findings in https://github.com/phan/phan/issues/2651#issuecomment-483329034