它不起作用，因为你没有传递一个数组给这个：
in_array($user_data['email'], $match_email)
您需要将$match_email转换为具有explode()的数组。

<?php

$match_email = explode(', ', 'muskan2874@gmail.com, khushi7382@gmail.com');

$config = [
    'special_user' => [
        'muskan' => [
            'age' => '24',
            'email' => 'muskan2874@gmail.com'
        ],
        'khushi' => [
            'age' => '21',
            'email' => 'khushi7382@gmail.com'
        ]
    ]
];


foreach ($config as $name => $users) {
    foreach ($users as $author => $user_data) {
        if (in_array($user_data['email'], $match_email)) {
            echo 'Special User';
            break;
        }
    }
}

如果可能的话，您也可以首先使用数组：

<?php

$match_email = ['muskan2874@gmail.com', 'khushi7382@gmail.com'];

$config = [
    'special_user' => [
        'muskan' => [
            'age' => '24',
            'email' => 'muskan2874@gmail.com'
        ],
        'khushi' => [
            'age' => '21',
            'email' => 'khushi7382@gmail.com'
        ]
    ]
];


foreach ($config as $name => $users) {
    foreach ($users as $author => $email) {
        if (in_array($email['email'], $match_email)) {
            echo 'Special User';
            break;
        }
    }
}