简单的PHP MYSQL select语句不返回任何内容

Alright, so I have a simple database with one table, and I have a function which is supposed to get all the rows for that one table:

function get_days() {
    global $db;
    $query = 'SELECT * FROM days'
            . 'ORDER BY idDays';
    $statement = $db ->prepare($query);
    $statement ->execute();
    $the_days = $statement->fetchAll();
    //$statement->closeCursor();
    return $the_days;
    //return $statement;
}

I've checked everything else, everything else functions just fine, including the part of my site where I input data into the table, that insert statement works just fine, so I've narrowed it down to this one select statement.

The problem is in you SQL syntax. You should do this:

function get_days() {
    global $db;
    $query = 'SELECT * FROM days '
            . 'ORDER BY id';
    $statement = $db ->prepare($query);
    $statement ->execute();
    $the_days = $statement->fetchAll();
    //$statement->closeCursor();
    return $the_days;
    //return $statement;
}

The problem is the string concatenation of your query:

$query = 'SELECT * FROM days' . 'ORDER BY idDays';

This results in: SELECT * FROM daysORDER BY idDays

Include a space character instead:

$query = 'SELECT * FROM days' . ' ORDER BY idDays';

You can avoid problems like this with proper error handling:

try{
    $statement->execute();
}
catch(PDOException $e){
    exit($e->getMessage());
}

You might also want to remove the spaces in:

$db ->prepare($query);
$statement ->execute();

So they become:

$db->prepare($query);
$statement->execute();

This is simple way to select you can use a function for it.

$servername = "localhost";
$username = "username";
$password = "password";
$dbname = "myDB";

// Create connection
$conn = new mysqli($servername, $username, $password, $dbname);
// Check connection
if ($conn->connect_error) {
    die("Connection failed: " . $conn->connect_error);
} 

        $sql = "SELECT * FROM days ORDER BY idDays";
        $result = $conn->query($sql);

        if ($result->num_rows > 0) {
            // output data of each row
            while($row = $result->fetch_assoc()) {
                //do  anything
            }
        } else {
            echo "0 results";
        }