如何判断php mysqli扩展类是否开启

<?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>"; }?>