The function str_replace() (as defined in PHP manual) – “replace all occurrences of the search string with the replacement string.
What this means if you simply need to replace a character in a variable or a word in a phrase you should use this function. (If you need to use regular expressions to find a pattern match then use preg_replace() instead).
$text = 'I like apples.';
$new_text = str_replace('like', 'hate', $text);
echo $new_text; // will print 'I hate apples.'
As simple as that.