อยากถามเรื่องการตัดสตริงหัว-ท้าย ตามอักขระที่กำหนดครับ

<?php
echo substr("abcdef", -1)."<br>";    // returns "f"
echo substr("abcdef", -2)."<br>";    // returns "ef"
echo substr("abcdef", -3, 1)."<br>"; // returns "d"
?>

<?
$str="The bird flys in through the windows.

//The cat jumps up onto the bed.

The dog is black."; 

/*ผมอยากจะให้เหลือแค่

"The bird flys in through the windows.

The dog is black."
*/

$pos= strpos($str,'//');
$sub=substr($str,$pos,-17);
$result=str_replace($sub,'',$str);

echo $result;
?>

<?php
$s = "The bird flys in through the windows.\r\n//The cat jumps up onto the bed.\r\n//The rat jumps up onto the table.\r\nThe dog is black.//The rat jumps up onto the table.\r\n";
echo '<br>bfore:<pre>',$s,'</pre>';
	$s = preg_replace('/^(\/\/[^\r\n]*\r\n)/m','',$s);
echo '<br>after:<pre>',$s,'</pre>';
?>