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

Question

What does a \\ do in PHP?

For example, CSRF4PHP has \\FALSE, \\session_id, and \\Exception:

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Answer 1

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"