Important when using classes:
You must call db2_execute() in the same scope as where you define / set / bind the variables.
For example:

<?php
class DB2Class {
    public $conn;
    private $usr = 'user';
    private $pss = 'password';
    private $cat = 'catalog';
    public function db2_conn(){
        $conn = db2_connect($this->cat,$this->usr,$this->pss);
        if(!$conn)
            throw new Exception(db2_conn_errormsg());
        $this->conn = $conn;
    }
    public function db2_prep($sql){
        if(!$stmt = db2_prepare($this->conn, $sql)){
            throw new Exception($sql . " " . db2_stmt_errormsg());
            return false;
        }
        return $stmt;
    }
    public function db2_exec($stmt){
        if(!db2_execute($stmt))
            throw new Exception(db2_stmt_errormsg($stmt));
    }
}

/* This will NOT work */
function bindtest(){
    try {
        $db2 = new DB2Class;
        $db2->db2_conn();
        $stmt = $db2->db2_prep("SELECT * FROM TABLE WHERE FIELD = ?");
        $field = 'value';
        db2_bind_param($stmt, 1, "field", DB2_PARAM_IN);
        $db2->db2_exec($stmt);        // Results in Unbound Variable Error
        while($row = db2_fetch_assoc($stmt))
            var_dump($row);
    } catch(Exception $e) {
        error_log( $e->getMessage () );
    }
}

/* This will work */
function bindtest(){
    try {
        $db2 = new DB2Class;
        $db2->db2_conn();
        $stmt = $db2->db2_prep("SELECT * FROM TABLE WHERE FIELD = ?");
        $field = 'value';
        db2_bind_param($stmt, 1, "field", DB2_PARAM_IN);
        if(!db2_execute($stmt))
            throw new Exception(db2_stmt_errormsg($stmt));
        while($row = db2_fetch_assoc($stmt))
            var_dump($row);
    } catch(Exception $e) {
        error_log( $e->getMessage () );
    }
}

?>