One good example of such a coding standard is PSR-1, and it's extension PSR-2. Some of the rules of these standards are:
- Files MUST use only <?php and <?= tags.
- Files MUST use only UTF-8 without BOM for PHP code.
- Files SHOULD either declare symbols (classes, functions, constants, etc.) or cause side-effects (e.g. generate output, change .ini settings, etc.) but SHOULD NOT do both.
- Namespaces and classes MUST follow an "autoloading" PSR: [PSR-0, PSR-4].
- Class names MUST be declared in StudlyCaps.
- Class constants MUST be declared in all upper case with underscore separators.
- Method names MUST be declared in camelCase.
More details on these coding standards can be found here and here.