This can cause unpleasant problems when using a library function that cleans up after itself.

The script:

<?
  function some_reuseable_function() {
    // Pretend this function was included from another file
    $conn2 = mysql_connect('host', 'uid', 'pwd');
    echo "Function connection is: $conn2<br>\n";
    mysql_close($conn2);
  }

$conn1 = mysql_connect('host', 'uid', 'pwd');
echo "Main connection is: $conn1<br>\n";
some_reuseable_function();
mysql_close($conn1);
?>

The output:

Main connection is: 1
Function connection is: 1

Warning: 1 is not a MySQL link index in test.php3 on line 12