PHP语法错误：PdoConnect.php第5行意外出现'const'报错排查

解析错误：语法错误，意外出现'const'（T_CONST），预期为变量（T_VARIABLE），位于文件C:\PHP\OSPANEL\OSPanel\domains\newyearshop.ua\classes\PdoConnect.php第5行

PdoConnect.php 原代码 #

<?php

class PdoConnect {

    private const HOST = 'localhost';
    private const DB = 'newyear';
    private const USER = 'newyear';
    private const PASS = 'newyear';
    private const CHARSET = 'utf8';

    protected $DSN;
    protected $OPD;
    public $PDO;

    private function __construct() {
        
        $this->DSN = "mysql:host=" . self::HOST . ";dbname=" . self::DB . ";charset=" . self::CHARSET;

        $this->OPD = array(
            PDO::ATTR_ERRMODE => PDO::ERRMODE_EXCEPTION,
            PDO::ATTR_DEFAULT_FETCH_MODE => PDO::FETCH_ASSOC,
            PDO::ATTR_EMULATE_PREPARES => false,
        );

        $this->PDO = new PDO($this->DSN, self::USER, self::PASS, $this->OPD);
    }

问题根源是PHP版本不兼容：类常量的private访问修饰符是PHP 7.1才新增的特性，若当前使用的PHP版本低于7.1，就会触发该语法错误。

有两种可行解决方式：

• 方案1：升级PHP版本
  将PHP版本升级到7.1或更高，可直接保留原代码中private const的写法，无需修改代码。

• 方案2：修改代码适配低版本PHP
  去掉类常量前的private修饰符，PHP7.1之前的版本中，类常量默认允许类内部及子类访问，修改后的代码如下：

<?php

class PdoConnect {

    const HOST = 'localhost';
    const DB = 'newyear';
    const USER = 'newyear';
    const PASS = 'newyear';
    const CHARSET = 'utf8';

    protected $DSN;
    protected $OPD;
    public $PDO;

    private function __construct() {
        
        $this->DSN = "mysql:host=" . self::HOST . ";dbname=" . self::DB . ";charset=" . self::CHARSET;

        $this->OPD = array(
            PDO::ATTR_ERRMODE => PDO::ERRMODE_EXCEPTION,
            PDO::ATTR_DEFAULT_FETCH_MODE => PDO::FETCH_ASSOC,
            PDO::ATTR_EMULATE_PREPARES => false,
        );

        $this->PDO = new PDO($this->DSN, self::USER, self::PASS, $this->OPD);
    }

内容的提问来源于stack exchange，提问作者bodis_