if (session_id() === "") { session_start(); }

希望能有所帮助!

推荐的方法为版本的PHP >= 5.4.0, PHP 7, PHP 8

if (session_status() === PHP_SESSION_NONE) {
    session_start();
}

参考:http://www.php.net/manual/en/function.session-status.php

适用于PHP < 5.4.0版本

if(session_id() == '') {
    session_start();
}

您可以使用以下解决方案来检查PHP会话是否已经启动:

if(session_id()== '')
{
   echo"Session isn't Start";
}
else
{
    echo"Session Started";
}

你可以这样做，而且非常简单。

if (!isset($_SESSION)) session_start();

使用session_id()，如果没有设置，它将返回一个空字符串。它比检查$_COOKIE更可靠。

if (strlen(session_id()) < 1) {
    session_start();
}

基于@Meliza Ramos的回复(见第一个回复)和http://php.net/manual/en/function.phpversion.php，

行动:

如果不存在，定义PHP_VERSION_ID 根据PHP_VERSION_ID定义函数检查版本 定义函数为openSession()安全

只使用openSession()

    // PHP_VERSION_ID is available as of PHP 5.2.7, if our
    // version is lower than that, then emulate it
    if (!defined('PHP_VERSION_ID')) {
        $version = explode('.', PHP_VERSION);

        define('PHP_VERSION_ID', ($version[0] * 10000 + $version[1] * 100 + $version[2]));


        // PHP_VERSION_ID is defined as a number, where the higher the number
        // is, the newer a PHP version is used. It's defined as used in the above
        // expression:
        //
        // $version_id = $major_version * 10000 + $minor_version * 100 + $release_version;
        //
        // Now with PHP_VERSION_ID we can check for features this PHP version
        // may have, this doesn't require to use version_compare() everytime
        // you check if the current PHP version may not support a feature.
        //
        // For example, we may here define the PHP_VERSION_* constants thats
        // not available in versions prior to 5.2.7

        if (PHP_VERSION_ID < 50207) {
            define('PHP_MAJOR_VERSION',   $version[0]);
            define('PHP_MINOR_VERSION',   $version[1]);
            define('PHP_RELEASE_VERSION', $version[2]);

            // and so on, ...
        }
    }

    function phpVersionAtLeast($strVersion = '0.0.0')
    {
        $version = explode('.', $strVersion);

        $questionVer = $version[0] * 10000 + $version[1] * 100 + $version[2];

        if(PHP_VERSION_ID >= $questionVer)
            return true;
        else
            return false;

    }

    function openSession()
    {
        if(phpVersionAtLeast('5.4.0'))
        {
            if(session_status()==PHP_SESSION_NONE)
                session_start();
        }
        else // under 5.4.0
        {
            if(session_id() == '')
                session_start();
        }
    }