如何在循环之间添加一些信息？

$data = explode(",", $str);
$names = array();
for ( $i = 0; $i < count($data); $i+=3) {  
    $names[] = $data[$i];
}

Suppose this gives the result:

mathews,neil,ambrose,glen,ethan

I want add and just before ambrose so the final result is:

mathews,neil, and ambrose

You should avoid loops for this kind of operation, they add an unnecessary level of complexity to your code.

// put the last element of $data to $lastName, and remove the last element from $data
$lastName = array_pop($names);
echo join(',',$names) . ' and ' . $lastName;

There are also many one line variations like this:

// replace the last ",name" with " and name"
echo preg_replace('/(,)[a-z]+$/', ' and ', join(',', $names));

But they wouldn't be very clear to inexperienced PHP developers.

edit, If you are happy to ditch the array you already have in your example, you can do this.

$str = "mathews,neil,ambrose,glen,ethan";
$names = array_slice(explode(',', $str), 0, 3);
$lastName = array_pop($names);
echo join(',', $names) . ' and ' . $lastName;

Since you're already using a counter $i, you could just perform the following check:

if ($i === count($data) - 2) {
    echo ' and ';
}

...or whatever. It doesn't have to be echo.

$data = explode(",", $str);
for ($i = 0; $i<count($data)-1; $i++) {
   $sentence = $sentence.$data[$i].", ";
}
$sentence = $sentence."and ".end($data).".";
echo $sentence;

What this will do is store all of the names except for the last one in a variable, with commas, and then at the end adds the last one with "and" before it.

That variable - $sentence - is then echoed (displayed).

EDIT: To only display the first three results is simple.

$data = explode(",", $str);
echo $data[0].", ".$data[1].", and ".$data[2];

The $data[x] simply returns one of the names, which are stored in order.