Description:
------------
A common pattern in PHP is to see str_replace being used with array_keys and 
array_values to replace in a string according to the keys and values of an 
associative array. See comment of Wes Foster: http://php.net/str_replace#95198

It would be nice if PHP would provide a shorthand function for this, which will 
probably also provide an speed bonus.

Could be implementend as a seperate function or as different arguments, similar to  
strstr.

Test script:
---------------
/**
* Replace keys of associative array in string with values of associative array
* @param string[] $replacing Key => value pairs of replacements
* @param string $subject The haystack
* @see str_replace()
**/
function str_replace_assoc($replacing, $subject) {
    return str_replace(array_keys($replacing), array_values($replacing), $subject);    
}

// Provides: You should eat pizza, beer, and ice cream every day
$phrase  = "You should eat fruits, vegetables, and fiber every day.";
$replacements = ["fruits" => "pizza", "vegetables" => "beer", "fiber" => "ice cream"];

echo str_replace($replacements, $phrase);

Expected result:
----------------
You should eat pizza, beer, and ice cream every day