What does a \ (backslash) do in PHP (5.3+)?

Namespaces

In PHP 5.3+ the backslash \ symbol is used in namespaces. It is the start symbol to indicate a namespace and also serves as a separator between sub-namespace names.

See official documentation about namespacing.

Opcache

Additionally in PHP 7.0+ some functions are replaced with opcodes by OPCache, which makes these specific functions run a lot faster. However this only works when the functions are placed in the root namespace. See this discussion about this topic. So besides namespacing, the \ indirectly also affects code optimisation.

The following native functions benefit from this effect:

"array_slice"
"assert"
"boolval"
"call_user_func"
"call_user_func_array"
"chr"
"count"
"defined"
"doubleval"
"floatval"
"func_get_args"
"func_num_args"
"get_called_class"
"get_class"
"gettype"
"in_array"
"intval"
"is_array"
"is_bool"
"is_double"
"is_float"
"is_int"
"is_integer"
"is_long"
"is_null"
"is_object"
"is_real"
"is_resource"
"is_string"
"ord"
"strlen"
"strval"

The \ is used in PHP 5.3 for namespaces. See http://www.php.net/manual/en/language.namespaces.rationale.php for more information on namespaces and PHP.

To clarify potential confusion:

The backslash does not imply class inheritance.

In the following, Animal, Dog, Shepherd don't have to be classes, but simply namespaces. Meaning something used to group names together to avoid naming collisions.

$myDog = new \Animal\Dog\Shepherd\GermanShepherd();

The leading \ means Animal was declared in the global scope.

\ (backslash) is the namespace separator in PHP 5.3.

A \ before the beginning of a function represents the Global Namespace.

Putting it there will ensure that the function called is from the global namespace, even if there is a function by the same name in the current namespace.