How do I creat search from multi input field

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PHP

MySQL

How to create search in a table on database from multi input field
i have a table with many columns some of this is category subcategory evaluation. ..
user select from a drop down category a evaluation 5 all categories a with evolution 5 should appear if select other input like subcategory the search is more specifie and this drop down have data from database I try to do this but no results can you help me this my code

What I have tried:

<?php
if(isset($_POST['submit'])) {         
    $RelatedCustome=$_POST['txt_RelatedCustomer'];
    $LevelofPriority=$_POST['txt_LevelofPriority'];
    $category=$_POST['txt_category'];
    $subcategories=$_POST['txt_subcategories'];
    $ProductOrService=$_POST['txt_ProductOrService'];
    $CompanyName=$_POST['txt_CompanyName'];
    $Namecontactperson=$_POST['txt_Namecontactperson'];
    $evaluation=$_POST['txt_evaluation'];
    $sql=mysqli_query($conn,"select * from tbl_subcontractor where db_RelatedCustome='$RelatedCustome' or db_LevelofPriority='$LevelofPriority' or db_Category='$category' or db_Subcategories='$subcategories' or db_ProductOrService='$ProductOrService' or db_CompanyName='$CompanyName' or db_Namecontactperson='$Namecontactperson' or db_evaluation='$evaluation'" )or die(mysqli_error($conn));
    echo"<table class>";
echo"<table><tbody><tr>";
  echo" <td style>Related Customer
        <td style>Level of Priority
        <td style>Category
        <td style>Subcategory
        <td style>Product Or Service
        <td style>Company Name
        <td style>Name contact person
        <td style>Evaluation
        ";
 echo"</tr></tbody></table>";

    while($res=mysqli_fetch_array($sql)){
    $RelatedCustome=$res['db_RelatedCustome'];
    $LevelofPriority=$res['db_LevelofPriority'];
    $category=$res['db_Category'];
    $subcategories=$res['db_Subcategories'];
    $ProductOrService=$res['db_ProductOrService'];
    $CompanyName=$res['db_CompanyName'];
    $Namecontactperson=$res['db_Namecontactperson'];
    $evaluation=$res['db_evaluation'];
    echo"<table><tbody><tr>";
echo"<td >"; echo $RelatedCustome; echo"";
echo"<td >"; echo $LevelofPriority; echo"";
echo"<td>"; echo  $category; echo"</td>";
echo"<td >"; echo $subcategories; echo"";
echo"<td >"; echo $ProductOrService; echo"";
echo"<td >"; echo $CompanyName; echo"";
echo"<td >"; echo $Namecontactperson; echo"";
echo"<td>"; echo $evaluation; echo"</td>";
echo"</tr></tbody></table>";
        } echo"";
     }