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PHPMaster.com: Constructors and the Myth of Breaking the Liskov Substitution Principle

Note: This article was originally published at PHPDeveloper on 9 October 2012.
PHPDeveloper

On PHPMaster.com there's a new post in a series looking at the SOLID design principles in PHP development. In this new tutorial they try to dispel the myth that constructors break the Liskov Substitution Principle ("L" in "SOLID").

Rants aside, the morale of the story can be boiled down to the following: "Object construction is not part of the contract honored by its implementers". It's easier to grasp concepts by example rather than reading dull theory, so in this article I'll be demonstrating from a practical standpoint how the implementation of different constructors down the same hierarchy isn't a violation of the Liskov Substitution Principle, which is a reason why you shouldn't fall into the temptation of tainting your interfaces with constructors.

He illustrates the point with a simple PDO class that implements a "DatabaseAdapterInterface" interface that defines its own constructor that follows the defaults of the PDO extension. He goes on and changes the constructor for the class a bit to take in an array of config options rather than the DSN/User/Password combo. Inside of this constructor, those values are then taken and pushed into PDO to create the connection. He also suggests one other solution - the injection of a connection object ("ConnectionDefinition") into the constructor instead of the configuration directly.