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Using namespaces: fallback to global function/constant

Inside a namespace, when PHP encounters a unqualified Name in a class name, function or constant context, it resolves these with different priorities. Class names always resolve to the current namespace name. Thus to access internal or non-namespaced user classes, One must refer to them with their fully qualified Name as in:

Example #1 Accessing global classes inside a namespace

 <?php
namespace A\B\C;
class Exception extends \Exception {}

$a = new Exception('hi'); // $a is an object of class A\B\C\Exception
$b = new \Exception('hi'); // $b is an object of class Exception

$c = new ArrayObject; // fatal error, class A\B\C\ArrayObject not found
?>

For functions and constants, PHP will fall back to global functions or constants if a namespaced function or constant does not exist.

Example #2 global functions/constants fallback inside a namespace

 <?php
namespace A\B\C;

const E_ERROR = 45;
function strlen($str)
{
    return \strlen($str) - 1;
}

echo E_ERROR, "\n"; // prints "45"
echo INI_ALL, "\n"; // prints "7" - falls back to global INI_ALL

echo strlen('hi'), "\n"; // prints "1"
if (is_array('hi')) { // prints "is not array"
    echo "is array\n";
} else {
    echo "is not array\n";
}
?>