Create a function with three parameters (text, letter, replacement), and you will find it in the text given by you in the text parameter letter that will be entered by you in the letter parameter and replaced with replacement parameter code example

str_replace ($search, $replace, $subject);

<?php
$string = str_ireplace("FoX", "CAT", "the quick brown fox jumps over the lazy dog");
echo $string; // the quick brown CAT jumps over the lazy dog
?>