How can I swap the values of two different characters in a string with PHP? A becomes B, B becomes A [duplicate]

Question

I have a string in PHP. I want to swap a certain character with the value from another character. If I do it my way, A becoming B will replace A with B but the already existing B values will remain the same. When I try to swap B into A, there are, of course, values that were not originally swapped, because they were already there.

I tried this code.

$hex = "long_hex_string_not_included_here";
$hex = str_replace($a,$b,$hex);
//using this later will produced unwanted extra swaps
$hex = str_replace($b,$a,$hex);

I am looking for a function to swap these values.

Answer 1

Just use strtr. It's what it was designed for:

$string = "abcbca";
echo strtr($string, array('a' => 'b', 'b' => 'a'));

Output:

bacacb

The key functionality that helps here is that when strtr is called in the two argument form:

Once a substring has been replaced, its new value will not be searched again.

This is what stops the a that was replaced by b then being replaced by a again.

Demo on 3v4l.org

Answer 2

We could just try to substitute some third intermediate value for B, then replace all A to B, and then replace the marker back to A. But this always leaves open the possibility that the marker character might already appears somewhere in the string.

A safer approach is to covert the input string into an array of characters, and then walk down the array simply checking each index for A or B, and swapping accordingly.

$input = "AzzzzB";
echo $input ."\n";
$arr = str_split($input);

for ($i=0; $i < count($arr); $i++) {
    if ($arr[$i] == 'A') {
        $arr[$i] = 'B';
    }
    else if ($arr[$i] == 'B') {
        $arr[$i] = 'A';
    }
}
$output = implode('', $arr);
echo $ouput;

AzzzzB
BzzzzA

Note also that this approach is efficient, and only requires walking down the input string once.