php单词字符串怎么转为数组

$str = "This is a sample string";$word_array = explode(" ", $str);print_r($word_array);

Array(    [0] => This    [1] => is    [2] => a    [3] => sample    [4] => string)

$str = "This is a sample string. It contains punctuation!";$word_array = preg_split('/[s,.!]+/', $str);print_r($word_array);

Array(    [0] => This    [1] => is    [2] => a    [3] => sample    [4] => string    [5] => It    [6] => contains    [7] => punctuation)

$str = "This is a sample string.
 It contains white space.  ";$word_array = preg_split('/[s,.!]+/', $str);for ($i=0; $i<count($word_array); $i++) {    $word_array[$i] = trim($word_array[$i]);}print_r($word_array);

Array(    [0] => This    [1] => is    [2] => a    [3] => sample    [4] => string    [5] => It    [6] => contains    [7] => white    [8] => space)