Type Invariant¶

A type invariant is a data container whose type can’t change between a parent class and a child class. This is the case of properties.

The child type must be a subtype of the parent type and the parent type must be a subtype of the child type. This implies that the types are equal.

Invariance is the third type of variance, with contravariance and covariance.

<?php

class p {
     protected string $p;
}

class q extends p {
     // same a above
     protected string $p;
}

?>

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