Your line below:
$labelCPU = explode(':',str_replace('_',' ',$row['itemName']));
Is changing it into an array. Explode basically takes a string and converts it to an array splitting it by the character you specify (in this case ':').
You need to do something like:
for ($i=0; $i<count($labelCPU); $i++) {
$aa .= "<option value='{$row['partID']}'>$labelCPU[$i]</option>";
}
I suspect you are using explode to get the category name after ":". So perhaps this would work better:
$aa .= "<option value='{$row['partID']}'>$labelCPU[1]</option>";