how to return value in PHPrecursive function - php

Question

i am trying to build a recursive tree but child function is not returning any value. i don't know how to return value in PHP code. is there i am doing any mistake. please help
$sql="select * from MenuMaster order by MenuId";
$result= ExecuteQuery($sql);
function BuildMenu($result){
$output ='';
$output .= '<Ul>';
while ($row = mysqli_fetch_array ($result)){
if($row['ParentId']== null || $row['ParentId'] ==0){
$output .= '<li>'.$row['MenuText'].'</li>';
}
$output .= FindChild($output , $row['MenuId']);
}
$output.= '</Ul>';
return $output;
}
$finaloutput.= BuildMenu($result);
echo $finaloutput;
function FindChild($childOutput, $Menuid){
$sqlChild="select * from MenuMaster where ParentId = $Menuid ";
$resultChild= ExecuteQuery($sqlChild);
$Rowcont =mysqli_num_rows($resultChild);
if($Rowcont > 0){
$childOutput.='<Ul>';
while ($row1 = mysqli_fetch_array ($resultChild)){
$childOutput.= '<li>'.$row1['MenuText'].'</li>';
FindChild($childOutput,$row1['MenuId']);
}
$childOutput.='</Ul>';
}
return $childOutput;
}