Selective variable declaration in php

I don't know how to selectively declare variables in php. let's look at my process first.
Start:

$ Field = ['A', 'BB ', ......]; # dozens of 20 + $ data = array (); foreach ($ field as $ v) {switch ($ v) {case "aa ": $ data [$ v] = get_aa (); break ;.......}}

Later
If $ field has only one value zz
In this way, the program will compare and judge from the first case one by one, which is not good.
Then I will

$ Field = ['A', 'BB ', ......]; # $ aa = function () {return get_aa () ;};# add the operation code to the anonymous function $ data = array (); foreach ($ field as $ v) {$ data [$ v] =$ $ v ();}

# This frees you from a lot of judgment and the problem arises.
# In this case, many variables such as $ aa and $ bb need to be declared in advance, so I will wonder if $ field = ['zz '] can I choose to load only $ zz?
Or you can simply get the most primitive judgment for no reason. what kind of thinking do you have not figured out?
Thanks

Reply to discussion (solution)

You run get_aa () on all the members of the array $ field ()
There will still be get_aa (), get_bb (), get_cc ()....
For the latter $ {'get _ '. $ v} (),

More general

$f = array(  'a' => function() { return 'A'; },  'b' => function() { return 'B'; },  'c' => function() { return 'C'; },  'd' => function() { return 'D'; },);$d = array('c', 'b');foreach(array_intersect_key($f, array_flip($d)) as $k=>$v) {  $r[$k] = $v();}print_r($r);

Array(    [b] => B    [c] => C)

You run get_aa () on all the members of the array $ field ()
There will still be get_aa (), get_bb (), get_cc ()....
For the latter $ {'get _ '. $ v} (),

Thanks for your reply

The program algorithms corresponding to each field are different and irregular.

Yes

$f = array(  'a' => function() { return 'A'; },  'b' => function() { return 'B'; },  'c' => function() { return 'C'; },  'd' => function() { return 'D'; },);$d = array('c', 'b');$r = array_intersect_key($f, array_flip($d));array_walk($r, function(&$v) { $v = $v(); });print_r($r);

Array(    [b] => B    [c] => C)

Yes

$f = array(  'a' => function() { return 'A'; },  'b' => function() { return 'B'; },  'c' => function() { return 'C'; },  'd' => function() { return 'D'; },);$d = array('c', 'b');$r = array_intersect_key($f, array_flip($d));array_walk($r, function(&$v) { $v = $v(); });print_r($r);

Array(    [b] => B    [c] => C)

Thanks for your advice
This array plays 6
If there is a problem, the array $ f in the actual application will be very large, and every value is an anonymous function, does it occupy memory? I think too much, you don't have to worry about it.

So do you think writing static functions one by one will not occupy the memory?

If you really need to dynamically execute static functions, writing anonymous functions is the best choice.
Not only can compilation overhead be reduced (if there are only 10 static functions, then 10 thousand are also compiled)
In addition, the function does not need to be named and is directly bound to the purpose, saving the corresponding check.

So do you think writing static functions one by one will not occupy the memory?

If you really need to dynamically execute static functions, writing anonymous functions is the best choice.
Not only can compilation overhead be reduced (if there are only 10 static functions, then 10 thousand are also compiled)
In addition, the function does not need to be named and is directly bound to the purpose, saving the corresponding check.

Thank you, boss,
That's it.