Description:
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From manual page: http://www.php.net/function.mysqli-connect
---

$con=mysqli_connect("localhost","my_user","my_password","my_db");

var_dump($con);  -> Return mysql object.

$conn = mysqli_connect(NULL, $user, $pass);
$conn = mysqli_connect('NULL', $user, $pass);
Return Same/old object given before. Not thrown false or error.

$conn = mysqli_connect('NULLTest', $user, $pass);
Return false

Test script:
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$con=mysqli_connect("localhost","my_user","my_password","my_db");

var_dump($con);  -> Return mysql object.

$conn = mysqli_connect(NULL, $user, $pass);
$conn = mysqli_connect('NULL', $user, $pass);
Return Same object given before. Not thrown false or error.

$conn = mysqli_connect('NULLTest', $user, $pass);
Return false

Expected result:
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$conn = mysqli_connect(NULL, $user, $pass);

Should return false.

Actual result:
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$conn = mysqli_connect(NULL, $user, $pass);

var_dump($con);  -> Return mysql object.