在document.ready中声明的函数是未定义的？(function declared inside document.ready is undefined?)

 您可以根据$multiple_names_array的大小制作占位符。 您不应该像在代码中那样直接将值放入查询中，因为（如注释中所述）您很容易受到SQL注入的影响。 我也将所有的值放入一个数组中。 
 
 这是您正在寻找的解决方案。 所有值都绑定 。 
 
//Assuming the names are separated by commas, e.g "John,Jack,Jill';
$multiple_names = $_POST['multiple_names'];  
$multiple_names_array = explode(',',$multiple_names);

$placeholders = implode(', ',  array_fill(0, count($multiple_names_array), '?'));

//Put all values into one array.
$arguments = array_merge(array(), $multiple_names_array);
array_push($arguments, $checkInDateFinal);
array_push($arguments, $checkOutDateFinal);

// SELECT * FROM attendance WHERE name IN (?, ?, ?, ?, etc...) AND today_date BETWEEN ? AND ? ORDER BY id ASC
$stmt1 = $conn->prepare("SELECT * from attendance where name in ($placeholders) and today_date between ? and ? order by id ASC");
$stmt1->execute($arguments);

You can make placeholders depending on the size of $multiple_names_array. You should never put values directly into a query like in your code because (as said in the comments) you are vulnerable to SQL injection. I would also put all of the values into one array. 
 
This is the solution you are looking for. All values are binded.
 
//Assuming the names are separated by commas, e.g "John,Jack,Jill';
$multiple_names = $_POST['multiple_names'];  
$multiple_names_array = explode(',',$multiple_names);

$placeholders = implode(', ',  array_fill(0, count($multiple_names_array), '?'));

//Put all values into one array.
$arguments = array_merge(array(), $multiple_names_array);
array_push($arguments, $checkInDateFinal);
array_push($arguments, $checkOutDateFinal);

// SELECT * FROM attendance WHERE name IN (?, ?, ?, ?, etc...) AND today_date BETWEEN ? AND ? ORDER BY id ASC
$stmt1 = $conn->prepare("SELECT * from attendance where name in ($placeholders) and today_date between ? and ? order by id ASC");
$stmt1->execute($arguments);