strip_tags("hello <p> world</p>")–> “hello word”
compact("variablename1","variablename2",...") returns an array whose members are
"variablename1"=>$variablename1, "variablename2"=>$variablename2, …. if $variablename is not in the current scope, compact won’t include it.
array_rand($arr) randomly picks up an entry from the array $arr and returns its key.
array_count_values($arr) counts the frequency of values of $arr and return an array with values as the keys and their frequencies as the values.
preg_match_all($pattern,$str,$matches) finds all matches of $pattern in $str, and puts the results to $matches. $matches stores all matches of the pattern. $matches stores all strings that matched the first parenthesized subpattern, $matches stores all strings that matched the second parenthesized subpattern, and so on.