ALTER PROCEDURE test_proc 
 @in1 as nvarchar(max),
 @out2 int OUTPUT
AS
begin
 SELECT @out2 = len(@in1);
end

SQLSRV_PARAM_IN,
SQLSRV_PHPTYPE_STRING('UTF-8'),
SQLSRV_SQLTYPE_NVARCHAR(4001)

SQLSTATE: IMSSP
code:     -31
message:  An invalid size or precision for parameter 1 was specified.

SQLSRV_SQLTYPE_NVARCHAR('max')

$str1 = '...6000+ символов...';
$str2 = 0;
$params = [
    [
        &$str1,
        SQLSRV_PARAM_IN,
        SQLSRV_PHPTYPE_STRING('UTF-8'),
        SQLSRV_SQLTYPE_NVARCHAR(4001)
    ],
    [
        &$str2,
        SQLSRV_PARAM_INOUT,
        SQLSRV_PHPTYPE_INT,
        SQLSRV_SQLTYPE_INT
    ]
];
$sql = "{call test_proc (?,?)}";