如何查找仅在字符串中出现一次的数字？

Let's say I have input string

String 1 : 1234123
String 2 : 76523752

From those string, I'd like to know which numbers that only appears once. So in my case the output should be

String 1 : array(4)
String 2 : array(6,3)

I'm guessing I'll use strpos() for this task but I couldn't get any logical flow to make this happen. Can you help me figure this out?

Boring solution but it works!

$string="1234123";
$result=array();
foreach (count_chars($string, 1) as $i => $val) {
     if($val==1) $result[]=chr($i);
}
print_r($result);

The solution using array_keys, array_filter, array_count_values and str_split functions:

$strings = [
    'String 1' => '1234123',
    'String 2' => '76523752'
];

$result = [];
foreach ($strings as $k => $v) {
    $result[$k] = array_keys(array_filter(array_count_values(str_split($v)), function($v){
        return $v == 1;
    }));
}

print_r($result);

The output:

Array
(
    [String 1] => Array
        (
            [0] => 4
        )

    [String 2] => Array
        (
            [0] => 6
            [1] => 3
        )
)

As an alternative:

$string = "1235235236";
$res=[];
foreach(str_split($string) as $c) $res[$c]++; 
$res = array_filter($res, function($a){return $a == 1;}); 
var_dump($res);