PHP 函数 implode

<?php
   $a1 = array("1","2","3");
   $a2 = array("a");
   $a3 = array();

   echo "a1 is: '".implode("','",$a1)."'<br>";
   echo "a2 is: '".implode("','",$a2)."'<br>";
   echo "a3 is: '".implode("','",$a3)."'<br>";
?>