Replace first character of sting in PHP with rules - php

Question

I'm wanting to replace the first character of a string with a specific character depending on its value,
A = 0
B = 1
C = 2
Is there a way to do this based on rules? In total I will have 8 rules.
Ok, so I'm editing this to add more information as I don't think some people understand / want to help without the full picture...
My string will be any length between 5 and 10 characters
Capitals will not factor into this, it is not case sensitive
Currently there is no code, I'm not sure the best way to do this. I can write an if statement on a substring, but I know straight away that is inefficient.
Below is the before and after that I am expecting, I have kept these examples simple but all I am looking to do is replace the first character with a specific character depending on its value. For now, there are eight rules, but this could grow in the future
INPUT OUTPUT
ANDREW 1NDREW
BRIAN 2RIAN
BOBBY 2OBBY
CRAIG 3RAIG
DAVID 4AVID
DUNCAN 4UNCAN
EDDIE 5DDIE
FRANK 6RANK
GEOFF 7EOFF
GIANA 7IANA
HAYLEY 8AYLEY
So as you can see, pretty straight forward, but is there a simple way to specifically specify what a character should be replaced by?

Answer 1

Assuming all the rules are for single characters, like in the example, it would be easisest to code them in to a dictionary:
$rules = array('A' => 0, 'B' => 0 /* etc... */);
$str[0] = $rules[$str[0]];

Answer 2

I think this is what you want.
<?php
$input = array('ANDREW','BRIAN','BOBBY','CRAIG','DAVID','DUNCAN','EDDIE','FRANK','GEOFF','GIANA','HAYLEY');
$array = range('A','Z');
$array = array_flip(array_filter(array_merge(array(0), $array)));
$output = [];
foreach($input as $k=>$v){
$output[] = $array[$v[0]].substr($v, 1);
}
print_r($output);
?>
Output:
Array (
[0] => 1NDREW
[1] => 2RIAN
[2] => 2OBBY
[3] => 3RAIG
[4] => 4AVID
[5] => 4UNCAN
[6] => 5DDIE
[7] => 6RANK
[8] => 7EOFF
[9] => 7IANA
[10] => 8AYLEY
)
DEMO: https://3v4l.org/BHLPk