php Replacing multiple spaces with a single space [duplicate]

Use preg_replace() and instead of [ \t\n\r] use \s:

$output = preg_replace('!\s+!', ' ', $input);

From Regular Expression Basic Syntax Reference:

\d, \w and \s

Shorthand character classes matching
digits, word characters (letters,
digits, and underscores), and
whitespace (spaces, tabs, and line
breaks). Can be used inside and
outside character classes.