Change case of every nth letter in a string in PHP

Question

I am trying to write a simple program which takes every 4th letter (not character) in a string (not counting spaces) and changes the case to it's opposite (If it's in lower, change it to upper or vice versa).

What I have so far:

echo preg_replace_callback('/.{5}/', function ($matches){
            return ucfirst($matches[0]);   
     }, $strInput);

Expected Result: "The sky is blue" should output "The Sky iS bluE"

Answer 1

$str = 'The sky is blue';
    $strArrWithSpace = str_split ($str);
    $strWithoutSpace = str_replace(" ", "", $str);
    $strArrWithoutSpace = str_split ($strWithoutSpace);
    $updatedStringWithoutSpace = '';
    $blankPositions = array();
    $j = 0;
    foreach ($strArrWithSpace as $key => $char) {
        if (empty(trim($char))) {
            $blankPositions[] = $key - $j;
            $j++;
        }
    }

    foreach ($strArrWithoutSpace as $key => $char) {
        if (($key +1) % 4 === 0) {
            $updatedStringWithoutSpace .= strtoupper($char);
        } else {
            $updatedStringWithoutSpace .= $char;
        }

    }

    $arrWithoutSpace = str_split($updatedStringWithoutSpace);
    $finalString = '';
    foreach ($arrWithoutSpace as $key => $char) {
        if (in_array($key, $blankPositions)) {
            $finalString .= ' ' . $char;
        } else {
            $finalString .= $char;
        }
    }

    echo $finalString;

Answer 2

Try this:

$newStr = '';
foreach(str_split($str) as $index => $char) {
    $newStr .= ($index % 2) ? strtolower($char) : strtoupper($char);
}

it capitalize every 2nd character of string

Answer 3

<?php
    $str = "The sky is blue";
    $str = str_split($str);
    $nth = 4; // the nth letter you want to replace
    $cnt = 0;
    for ($i = 0; $i < count($str); $i++) {
        if($str[$i]!=" " && $cnt!=$nth)
            $cnt++;
        if($cnt==$nth)
        {
            $cnt=0;
            $str[$i] = ctype_upper($str[$i])?strtolower($str[$i]):strtoupper($str[$i]);
        }
    }
    echo implode($str);
?>

This code satisfies all of your conditions.

Edit:

I would have used

$str = str_replace(" ","",$str);

to ignore the whitespaces in the string. But as you want them in the output as it is, so had to apply the above logic.