Importing classes and namespaces in PHP: What difference does a leading backslash make?

The manual specifies the backslash as unnecessary, which naturally means that if you still use it that the meaning is equivalent. However, as you have pointed out, the manual says that it is supposedly not allowed, which is false.

However, there is something else troubling with the manual. They advertise this:

// importing a global class
use \ArrayObject;

If it is true that import names are not processed relative to the current namespace, then use \ArrayObject and use ArrayObject must have the same meaning. What else could use ArrayObject refer to other than the global one? In practice, the engine will import the global one.

Also, with bugs such as this: http://bugs.php.net/bug.php?id=49143

I believe there is confusion over what the standard is supposed to be.

To answer your question: there is no difference. However, if I was the engine developer who was also a believer of the no-leading-slash standard, then I wouldn't need to consider a case where someone wrote use \Exception;. I believe this was likely the case.

In fact there is no difference in using leading backslash in importing namespaces at the moment and also information in PHP manual has changed:

Note that for namespaced names (fully qualified namespace names containing namespace separator, such as Foo\Bar as opposed to global names that do not, such as FooBar), the leading backslash is unnecessary and not recommended, as import names must be fully qualified, and are not processed relative to the current namespace.

So now there is true information that using leading backslash is not recommended but there is no info that it's not allowed at it was in past.

So at the moment:

use Exception;
use \Exception;

those 2 lines work the same but you should rather use the first one.

Usually the leading backslash defines, that the identifier is absolute. If its missing, the interpreter assumes, that it is a relative identifier.

This is an absolute identifier:

$x = new \Name\Space\To\Class();

This is a relative identifier, because of the no leading slash. It's relative to the current namespace:

namespace Name\Space;
$x = new To\Class;

This is also a relative identifier. In this case, its resolved against the use statement, because the last part (alias) is the same, as the first of the class:

namespace Other\Name\Space;
use Name\Space;
$x = new Space\To\Class;

However, because in namespace and use statements only absolute identifiers (fully qualified names) are allowed, it's ok to omit it here. In namespace, it's even not allowed to set the leading backslash.

For further information on how PHP resolves the different namespace declarations, see the namespace rules manual.