问获取字符串中的3个单词计数

EN

<?php
$string = "This is a sample text. It is a sample text used as an example.";
$arr = explode(' ',$string);
$i=0;
foreach($arr as $k=>$v)
{
echo $arr[$i]." ".$arr[$i+1]." ".$arr[$i+2]."<br>";
$i++;
}

This is a
is a sample
a sample text.
sample text. It
text. It is
It is a
is a sample
a sample text
sample text used
.....

$string = "This is a sample text. It is a sample text used as an example.";
$words = preg_split('/\W/', $string);
$wordsPerRow = 3;


$offset = 0;
while ($offset < count($words)) {
    print implode(' ', array_slice($words, $offset, $wordsPerRow)).PHP_EOL;
    $offset += $wordsPerRow;
}