Parameterised queries

I have a query which is created depending on which variables are submitted via a form, an easy example might be:
 
//start the query
$sql= "select tblaccounts.companyid
                ,tblaccounts.accountno
                ,tblmembers.memberno
                ,tblcards.cardno
from tblaccounts
WHERE tblmembers.companyid = '$companyid' ";
 
//build where clauses based on the existence of some variables
if (strlen($firstname)>0){
$sql.= " and tblmembers.firstname = '$firstname' ";}
 
if (strlen($surname)>0){
$sql.= " and tblmembers.surname = '$surname' ";}
 
if (strlen($postcode)>0){
$sql.= " and tblaccounts.postcode = '$postcode' ";            }
 
// complete the query
$sql.= "  and (tblcards.primarycard = 'primary') order by surname, firstname,  postcode";
 
//pass the parameters
$params=array($companyid, $firstname, $surname, $postcode);
 
//run the query
$result=sqlsrv_query($conn, $sql, $params);
 
At the moment I can only run the query using the proper variables, not the ? parameters as the parameter array will change depending on the variables required.

This me open to sql injections and problems with apostrophes etc within variable etc

Is there a way that I can use an associative array to store the possible varaibles and call them within the sql?

I am coding in php 5.6, using mssql on a windows server.