PHP Mysqli_stmt->bind_param报错的原因与解决办法

最近由于升级PHP版本导致wordpress的一个插件运行时报错，也没有其他插件可供替代。俗话说，自己动手，丰衣足食。于是我开始自己编写插件。插件需要用到数据库中的内容，于是我编写查询查询语句，使用了据说较为安全的mysqli::prepare方法。但是有一个报错困扰了我很久。

报错内容为“Fatal error: Uncaught Error: Call to a member function bind_param() on boolean in”，中文直译为“致命错误：未捕获错误：调用中布尔值上的成员函数bind_param（）”，问题代码如下：

$conn = new mysqli("db_host", "db_user", "db_pw", "db_name");
$stmt1 = $conn->prepare("SELECT col FROM tablename1 WHERE id=?");
$stmt1->bind_param("i", 1);
$stmt1->execute();
$stmt1->bind_result($col1);
while($stmt1->fetch()){
    $stmt2 = $conn->prepare("SELECT col FROM tablename2 WHERE id=?");
    $stmt2->bind_param("i", $col1);
    $stmt2->execute();
    $stmt2->bind_result(col2);
    ......
}
$stmt2->close();
$stmt1->close();
$conn->close();

$conn = new mysqli("db_host", "db_user", "db_pw", "db_name");

$stmt1 = $conn->prepare("SELECT col FROM tablename1 WHERE id=?");

$stmt1->bind_param("i", 1);

$stmt1->execute();

$stmt1->bind_result($col1);

while($stmt1->fetch()){

$stmt2 = $conn->prepare("SELECT col FROM tablename2 WHERE id=?");

$stmt2->bind_param("i", $col1);

$stmt2->execute();

$stmt2->bind_result(col2);

......

}

$stmt2->close();

$stmt1->close();

$conn->close();

问题发生在上述代码的8行，也就是bind_param语句。阅读报错得知，错误原因未捕获，也就是说，虽然程序在执行到第8行时发现了问题，但是问题不一定出在第8行。有可能是因为前边某些地方的参数有未被发现的错误，代码执行到第8行时才发现了错误，但是不知道错误究竟发生在哪里。而且，前边也有一个bind_param，并没有报错。

往前看，似乎没什么问题。猜测，一个PHP文件中只允许同时定义一个mysqli_stmt对象吗？于是我对代码进行了修改，关闭第一个mysqli_stmt对象后再开启第二个mysqli_stmt对象：

$conn = new mysqli("db_host", "db_user", "db_pw", "db_name"); 
$stmt1 = $conn->prepare("SELECT col FROM tablename1 WHERE id=?"); 
$stmt1->bind_param("i", 1); 
$stmt1->execute(); 
$stmt1->bind_result($col1);
if($stmt1->fetch()){
    $param = $col1;
}else{
    $param = false;
}
$stmt1->close();
if($param !== false){
    $stmt2 = $conn->prepare("SELECT col FROM tablename2 WHERE id=?");
    $stmt2->bind_param("i", $param);
    $stmt2->execute();
    $stmt2->bind_result(col2);
    ......
    $stmt2->close();
}
$conn->close();

$conn = new mysqli("db_host", "db_user", "db_pw", "db_name");

$stmt1 = $conn->prepare("SELECT col FROM tablename1 WHERE id=?");

$stmt1->bind_param("i", 1);

$stmt1->execute();

$stmt1->bind_result($col1);

if($stmt1->fetch()){

$param = $col1;

}else{

$param = false;

}

$stmt1->close();

if($param !== false){

$stmt2 = $conn->prepare("SELECT col FROM tablename2 WHERE id=?");

$stmt2->bind_param("i", $param);

$stmt2->execute();

$stmt2->bind_result(col2);

......

$stmt2->close();

}

$conn->close();

以上改写将先前的同时存在的stmt1和stmt2改写为了先结束一个再运行一个。

运行测试，成功！