Backward Compatible

A feature is backward compatible when it can be handled by a previous version than its version of publication. It is automatically ignored by the previous versions, either by design or by luck.

For example, the attributes #[Attribute] are backward compatible: they were introduced in version 8.0, and their syntax, starting as a comment with a #, is backward compatible: it is processed as a comment in previous PHP versions.

Note that this example is partially backward compatible: it may work or not.

The contrary of a backward compatible feature is backward incompatible.

<?php

    #[Attribute] // This is backward compatible
    class X {
        function foo(#[AttributeForParameter] $a) { // This is not backward compatible

        }
    }

?>

Documentation

See also Roave: BackwardCompatibilityCheck.

Related : Backward Incompatible, Forward Compatible, Legacy, Migration, Polyfill