Finally, i found the right way to do it..
$objPHPExcel = new PHPExcel;
$objSheet = $objPHPExcel->getActiveSheet();
//PROTECT THE CELL RANGE
$objSheet->protectCells('A1:B1', 'PHP');
// UNPROTECT THE CELL RANGE
$objSheet->getStyle('A2:B2')->getProtection()
->setLocked(PHPExcel_Style_Protection::PROTECTION_UNPROTECTED);
// PROTECT THE WORKSHEET SHEET
$objSheet->getProtection()->setSheet(true);
This is working perfectly!