php截取字符串

<?php
echo substr("Hello world",0,10)."<br>";  //Hello worl
echo substr("Hello world",1,8)."<br>";   //ello wor
echo substr("Hello world",0,5)."<br>";   
echo substr("Hello world",6,6)."<br>";

echo substr("Hello world",0,-1)."<br>";  //Hello worl
echo substr("Hello world",-10,-2)."<br>";  //ello wor
echo substr("Hello world",0,-6)."<br>";    //Hello
echo substr("Hello world",-2-3)."<br>";   //world
?>