Would like to add the reason.
In medium to big projects (when many developers are involved) this feature could be of a great help. Because the code could be written in self-documenting manner.

E.g. compare
  $html = formatSomeData( $data, true, false, true, 10 );
to
  $html = formatSomeData( entity_x: $data, italic: true, bold: false, monospace: true, fontSize: 10 );

In the second case one doesn't have to search for formatSomeData() definition to start using it.

There is no need to change the function declaration.  
Consider this syntax:
  printhello (bold => true, name => 'Martin');
and let the compiler translate it to 
  printhello ('Martin', true)
given the traditional declaration.

I would like to suggest the following syntax:

function f ($un1, $un2, $un3 = 'default',
 'hello' => 'goodbye', 'planet' => 'earth') {
    # $_FUNCARGS is a magic local short-hand for func_get_args()
    var_dump($_FUNCARGS);
}

When called as:

f('one', 'named' => 'param', 'two', 5 => 'works', 'hello' => 'world', 'too');

... the equivalent of this would happen behind the scenes:

# (The order and position of the *named* parameters here are just
# an example and not necessarily what PHP might implement)
$_FUNCARGS = array(0 => 'one', 1 => 'two', 2 => 'default', 5 => 'works',
 6 => 'too', 'named' => 'param', 'hello' => 'world', 'planet' => 'earth');
$un1 = &$_FUNCARGS[0];
$un2 = &$_FUNCARGS[1];
$un3 = &$_FUNCARGS[2];

The bottom line is that the parameters look like an array(), with
both numeric and string keys, in any order, just like an array.

For readability, developers should probably put the unnamed parameters
before the named parameters just as a matter of style (for both function
calls and declarations), but the PHP engine should take them as they come.

See #22216

helly@php.net said:
This has been discussed and declined several times. Check the mail archives for 
discussions.