PHP マニュアル: Similar to define(), but allows defining in class definitions as well

bool runkit7_constant_add(string $constant_name, mixed $value, int $newVisibility = ?)

constant_name: Name of constant to declare. Either a string to indicate a global constant, or classname::constname to indicate a class constant.
value: NULL, Bool, Long, Double, String, Array, or Resource value to store in the new constant.
newVisibility: Visibility of the constant, for class constants. Public by default. One of the RUNKIT7_ACC_* constants.

成功した場合に true を、失敗した場合に false を返します。