PSR-2 Section 4.2 Interpretation

Question

I was recently told by a colleague of mine that PSR-2 coding standard says that you are not allowed to use the "_" character to indicate whether or not the variable is private or protected.

He cited section "4.2 Properties" of http://www.php-fig.org/psr/psr-2/

Visibility MUST be declared on all properties.

The var keyword MUST NOT be used to declare a property.

There MUST NOT be more than one property declared per statement.

Property names SHOULD NOT be prefixed with a single underscore to indicate protected or private visibility.

I was outraged when I heard this because I am a big fan of the _ prefix on private and protected vars and I couldn't believe the community would accept such a standard.

My interpretation of this based on the "SHOULD NOT" keyword was that scope keywords MUST be used when declaring properties on a class and that it is recommended that you not use the _ character, but it is still allowed and you are still PSR-2 complaint if you choose to use this.

While I disagree with this and recommend that everyone prefix their private and protected vars with a underscore, I suspect the reasoning behind this is to prevent people from leaving out the scope keyword ( Public, Protected, Private ) and relying only the naming convention. This makes since, because as we all know with out the scope variable PHP makes everything Public.

http://www.ietf.org/rfc/rfc2119.txt

To summarize the question: Is the "_" prefix on private and protected variables on a class PSR-2 compliant?

Edit: Also, I'm not looking for a personal preference debate here, I just want to know if the use of the _ prefix is technically PSR-2 compliant.

Answer 1

I don't think that "I like it better" qualifies as particular circumstance where valid reasons exist, since the reason would then apply to your person, not the code base or the project.

I can think of some other examples that justify the SHOULD NOT:

• Project depends on some third-party tool, framework, whatever... that relies on names to identify visibility.

• Class method names need to mimic those from other project that use such convention (e.g., because it's a port from another language).

• Project was started before PHP/5. Renaming would break backwards compatibility.

Thus it isn't strictly forbidden, but it's clearly discouraged; probably, because it provides redundant information that can get on the way on further refactoring and can even be wrong:

// r12345 Expose HTML escaping
public function _escapeHtml(){
}

Answer 2

Let's combine PSR-2 and RFC 2119:

There may exist valid reasons in particular circumstances when prefixing property names with a single underscore is acceptable or even useful, but the full implications should be understood and the case carefully weighed before prefixing property names with a single underscore.

So to answer your question: Yes it is technically compliant. This is also reflected by phpcs --standard=PSR2 issuing only a warning:

WARNING | Property name "$_foo" should not be prefixed with an underscore to indicate visibility

AFAIR "_" comes from the times when PHP didn't had any visibility language construct. I have no idea what the intention about the PSR "_" policy was, but IMO it is very likely that the reason is that after introducing visibility there is no valid reason to use "_" anymore.