Compliler Error... Unexpected Char '''

In order to customize the effect of more then one button being held down at the same time, I have written a function called CheckButtons() that checks each frame to see which buttons are being held down, and then alters a variable named buttonsDown (string) which then can be accessed by other aspects of the script instead of the traditional Input.GetButton.... Here is the function:

function CheckButtons() { if(Input.GetButton("W")) { buttonsDown = buttonsDown + "w"; } if(Input.GetButton("A")) { buttonsDown = buttonsDown + "a"; } if(Input.GetButton("S")) { buttonsDown = buttonsDown + "s"; } if(Input.GetButton("D")) { buttonsDown = buttonsDown + "d"; }

 }

It is then accessed by the movement part of the script:

 if(buttonsDown = 'w')
{
    transform.position += transform.forward * moveSpeed * Time.deltaTime;
}

(Just one of many reactions to buttonsDown's current value.)

However, the pesky debugging console tells me that there is an unexpected character ''' in this line: if(buttonsDown = 'w'. I cannot seem to tell what error I made! I'm not that good with unity... so please be gentle if it's a really obvious mistake. Thanks!