PHP学习：如何判断php mysqli扩展类是否开启

PHP编程
<?php
 /* by http://www.manongjc.com/article/1206.html */
 function mysqlinstalled (){
  if (function_exists ("mysql_connect")){
   return true;
  } else {
   return false;
  }
 }
 function mysqliinstalled (){
  if (function_exists ("mysqli_connect")){
   return true;
  } else {
   return false;
  }
 }
 
 if (mysqlinstalled()){
  echo "<p>The mysql extension is installed.</p>";
 } else {
  echo "<p>The mysql extension is not installed..</p>";
 }
 if (mysqliinstalled()){
  echo "<p>The mysqli extension is installed.</p>";
 } else {
  echo "<p>The mysqli extension is not installed..</p>";
 }
?>