Error in Mysql update code using php

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PHP

I am having the following error in MySql update code in php when I run the script.

Database query failed because You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near '=

This is code causing the error:

public function update(){
     global $database;
  
     $sql = "UPDATE users SET ";
     $sql .= "FullName='".$database->mysqlPrep($this->fullName)."',";
     $sql .= "RoleID='". $database->mysqlPrep($this->roleID) ."', ";
     $sql .= "UserName='". $database->mysqlPrep($this->userName) ."',";
     $sql .= "Email='". $database->mysqlPrep($this->email) ."', ";
             $sql .="Password='". $database->mysqlPrep($this->password)."' ";
     $sql .= "WHERE ID=". $database->mysqlPrep($this->id);
     $database->query($sql);
     return ($database->affectedRows() == 1) ? true:false;
 }

The sql script in this update function gives the error when I invoke the function in html. One thing i find strange when debugging is if i take the script outside the function it works. So i can't quite wrap my head round what's going on here.

Php version = 5.4.3
MySql version = 5.5.24

I would appreciate any help. Thanks guys