php怎么去掉最后几个字符

<?php
header(&#39;content-type:text/html;charset=utf-8&#39;);
$str = &#39;123,234,345,&#39;;
echo "原字符串：",$str,&#39;<br/>&#39;;
echo "去掉最后1个字符的字符串：",substr($str,0,-1),&#39;<br/>&#39;;
echo "去掉最后2个字符的字符串：",substr($str,0,-2),&#39;<br/>&#39;;
echo "去掉最后3个字符的字符串：",substr($str,0,-3),&#39;<br/>&#39;;
echo "去掉最后4个字符的字符串：",substr($str,0,-4),&#39;<br/>&#39;;
echo "去掉最后5个字符的字符串：",substr($str,0,-5);
?>

原字符串：123,234,345,
去掉最后1个字符的字符串：123,234,345
去掉最后2个字符的字符串：123,234,34
去掉最后3个字符的字符串：123,234,3
去掉最后4个字符的字符串：123,234,
去掉最后5个字符的字符串：123,234

substr(string,start,length)