Change case of every nth letter in a string in PHP - php

Question

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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.