Creates a function, which accepts $args and returns the expression in $expression.

Items from the optional array $locals can be used as local variables in the function. In case of collision a formal arguments and a key in $locals, the latter will be prefixed with ext_

• string $args: String with the function arguments
• string $expression: String with an expression -- the result of the function
• array $locals: The items in this array will be extracted in the function as local variables.