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mysql extension is not installed in the web server PHP environment.
the mysql extension is no longer recommended and has been removed in PHP7.
http://www.php.net/manual/zh/.

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the message to report an error is to say

$_GET['columns'][$j]['searchable'];

$_ GET does not have the key columns (key)

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figure 2, the server-side PHP version is too high because the mysql_* series of functions have been completely removed from the PHP7, resulting in no errors in the functions.