I see the term legacy signature a lot in the documentation of programming languages. For example in the php documentation :

Passing the separator after the array (i.e. using the legacy signature) has been deprecated.

But I can’t figure out what it means. I googled it but the results seemed to have no correlation with programming.

  • The signature is a function's interface - how many parameters, what types, type of return value, etc.
    – jonrsharpe
    Commented May 21, 2022 at 15:29
  • 2
    You need to improve your google-fu. Some tips: if the most obvious search doesn't yield relevant results, try adding a qualifier ('legacy signature programming', 'legacy signature php'), if that doesn't work, try searching separately ('signature programming', 'what is signature in programming') or use google's "define" capability to see what the words themselves mean ('define legacy'). Commented May 22, 2022 at 14:32

2 Answers 2


Legacy just means something from the past. In this sort of context it means something from the past that is now disliked and should not be copied.

implode(array $array, string $separator): string is the legacy signature of the PHP implode function, because it was created in the past but the people in control of the PHP project no longer like it. You should not use it.


As mentioned already, legacy refers to remnants of the past. It has a generally negative connotation.

Signature refers to a method signature (which you'll find a lot of google hits for). In this case, they're referring to a particular's methods's combination of input parameters; which is part of that method's signature.

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