Php 如何检查数据库中是否存在电子邮件？

这是我的init.php

我不确定是否应该将_构造更改为公共函数，或者不将其调用到我的register.php中，在这里我试图查找输入空间的电子邮件是否在数据库中

$query = DB::__construct()->prepare("SELECT email FROM users WHERE email = ?" );
$query->bindValue( 1, $email );
$query->execute();

if( $query->rowCount() > 0 )  # If rows are found for query
{
echo "Email has already been registered";
}
else
{
echo "Email has not been registered before";
}

---

我将得到致命错误：未捕获错误：非静态方法DB:：_构造无法在C:\xampp\htdocs\mostwaint\register.php:4堆栈跟踪：0{main}中静态调用当我键入电子邮件时，在第4行的C:\xampp\htdocs\mostwaint\register.php中抛出。

私有构造函数表明类是作为单例模式实现的，在用于数据库访问的类中非常常见

---

寻找另一个公共方法，可能类似instance、getInstance、getConnection…

这样代码就不起作用了？什么部分？在register.php中。我非常确定DB的连接已经建立，但我不知道如何使用该连接访问电子邮件列，而不将私有功能更改为公共功能。

class DB {

private static $_instance = null;

private $_pdo, 
        $_query, 
        $_error = false, 
        $_results, 
        $_count = 0;

private function __construct() {

    try {

        $this->_pdo = new PDO(
            'mysql:host=' . 
            Config::get('mysql/host') . 
            ';dbname=' . Config::get('mysql/db'), 
            Config::get('mysql/username'), 
            Config::get('mysql/password'));

    } catch(PDOException $e) {

        die($e->getMessage());

    }

}

public static function getInstance() {

    if(!isset(self::$_instance)) {

        self::$_instance = new DB();

    }

    return self::$_instance;

}

public function query($sql, $params = array()) {

    $this->_error = false;

    if($this->_query = $this->_pdo->prepare($sql)) {

        $x = 1;

        if(count($params)) {

            foreach($params as $param) {

                $this->_query->bindValue($x, $param);
                $x++;

            }

        }

        if($this->_query->execute()) {

            $this->_results = $this->_query->fetchAll(PDO::FETCH_OBJ);
            $this->_count = $this->_query->rowCount(); 

        }

        else {

            $this->_error = true;

        }

    }

    return $this;

}

public function action($action, $table, $where = array()) {

    if(count($where) === 3) {

        $operators = array('=', '>', '<', '>=', '<=');

        $field      = $where[0];
        $operator   = $where[1];
        $value      = $where[2];

        if(in_array($operator, $operators)) {

            $sql = "{$action} FROM {$table} WHERE {$field} {$operator} ?";

            if(!$this->query($sql, array($value))->error()) {

                return $this;

            }

        }
    }
    return false;
}


public function get($table, $where) {

    return $this->action('SELECT *', $table, $where);

}

public function insert($table, $fields = array()) {

    $keys = array_keys($fields);
    $values = null;
    $x = 1;

    foreach($fields as $field) {

        $values .= "?";

        if($x < count($fields)) {

            $values .= ', ';

        }

        $x++;


        $sql = "INSERT INTO users (`" . implode('`,`', $keys) . "`) VALUES      ({$values})";

        if($this->query($sql, $fields)->error()) {

            return true;

        }

        echo $sql;

    }

    return false;

}

public function update($table, $id, $fields) {

    $set = '';
    $x = 1;

    foreach($fields as $name => $value) {

        $set .= "{$name} = ?";
        if($x < count($fields)) {

            $set .= ', ';

        }

        $x++;

    }

    $sql = "UPDATE {$table} SET {$set} WHERE user_id = {$id}";

    if(!$this->query($sql, $fields)->error()) {

        return true;

    }

    return false;

}

public function delete($table, $where) {

    return $this->action('DELETE', $table, $where);

}

public function results() {

    return $this->_results;

}

public function first() {

    return $this->results()[0];

}

public function error() {

    return $this->_error;

}

public function count() {

    return $this->_count;

}

}

$query = DB::__construct()->prepare("SELECT email FROM users WHERE email = ?" );
$query->bindValue( 1, $email );
$query->execute();

if( $query->rowCount() > 0 )  # If rows are found for query
{
echo "Email has already been registered";
}
else
{
echo "Email has not been registered before";
}