From the official doc here: https://github.com/prettier/plugin-php for Visual Studio Code: add php to document selectors in vscode settings (settings.json)

 "prettier.documentSelectors": [
    "**/*.{js,jsx,ts,tsx,vue,html,css,scss,less,json,md,mdx,graphql,yaml,yml,php}"
  ]

and declare the php parser in prettier config file (.prettierrc) https://prettier.io/docs/en/configuration

{
    "parser": "php",
    "plugins": ["@prettier/plugin-php"]
}

Core prettier does not support PHP¹, and thus neither does the plugin for VSCode/VSCodium. Luckily the designers of prettier seems to have thought of this and implemented a plugin system².

According to the prettier-vscode repo, all you need to do to use a plugin is to add it and prettier to your package.json³

So for php support your package.json would need to contain:

{
  "devDependencies": {
    "@prettier/plugin-php": "0.14.3",
    "prettier": "2.0.5"
  }
}