If pattern is an array and replacement is a string, then this replacement string is used for every value of pattern.
If pattern and replacement are arrays, then preg_replace() takes a value from each array and uses them to do search and replace on subject. If replacement has fewer values than pattern, then empty string is used for the rest of replacement values. If pattern is an array and replacement is a string, then this replacement string is used for every value of pattern. The converse would not make sense, though.