query msaccess database with php and update record

Hi, I am querying an MsAccess database with PHP
My code attached is working as specified, but I want to create a variable with the usuario (in this case "crescue")
If I replace "crescue" with the variable THEN IT DOESN'T WORK
Ex.
$abc = "crescue"
WHERE usuario = $abc
Why it does not work with the variable abc$

Also, how can I update the record once it is retrieved.  I would like to update the DIREC field for example.
A sample of how to do it with the code provided would be greatly appreciated.
Thanx again

<?php 
$db = 'C:\\Inetpub\\wwwroot\\locos\\DB\\bodega.mdb'; 
$conn = new COM("ADODB.Connection"); 
$conn->Open("Provider=Microsoft.Jet.OLEDB.4.0; Data Source=$db");
$abc = "crescue"
$sql = 'SELECT   usuario, nombre, contrasena, direc
      FROM     users WHERE usuario = "crescue" --> WORKS
      FROM     users WHERE usuario = $abc  --> DOESN'T       
      ORDER BY usuario'; 
$rs = $conn->Execute($sql); 
?> 
  
<table> 
<tr> 
      <th>Account #</th> 
      <th>Account Type</th> 
      <th>First Name</th> 
</tr> 
<?php while (!$rs->EOF) { ?> 
      <tr> 
            <td><?php echo $rs->Fields['usuario']->Value ?></td> 
            <td><?php echo $rs->Fields['nombre']->Value ?></td> 
            <td><?php echo $rs->Fields['contrasena']->Value ?></td> 
      </tr> 
      <?php $rs->MoveNext() ?> 
<?php } ?> 
</table> 
  
<?php 
  
$rs->Close(); 
$conn->Close(); 
  
$rs = null; 
$conn = null; 
  
?>

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