Default Value

Default value is the value that a parameter or a property takes when it is used for the first time, and have not been provided.

Arguments and properties may have a default value. That way, arguments may be skipped when the method is being called, and the properties do not have to be initialized before usage.

Variable have a default value of null. Their initial assignment is considered a default value, as it replaces the null one.

<?php

function foo($a = 1) {
    echo $a;
}

foo('one ');
foo();

// displays one 1

?>

Documentation

Related : Parameter, Properties, Default