$re = '/(7|\+7|8) \(\d{3}\) (\d)\2\2-\d\d-\d\d/m';
$str = '+7 (900) 000-90-90';
$subst = "\1";
$result = preg_replace($re, $subst, $str);
echo "The result of the substitution is ".$result;
Please keep in mind that these code samples are automatically generated and are not guaranteed to work. If you find any syntax errors, feel free to submit a bug report. For a full regex reference for PHP, please visit: http://php.net/manual/en/ref.pcre.php