php not recognizing string when passing to a js parameter

Question

I am currently working on a webiste and I am using php to connect to the database. I have a section where I want to have a html <button> tag that changes the src of the same <img> tag. so, i used javascript to make this (this was also from stackoverflow...)

    function changeImage(x)
    {
    var img = document.getElementById("change_image");
    img.src=x;

    }
    </script>

i want to have the parameter x, which is the name of the img (the src part. e.g. flower.jpg). inside the php code, i have a line echo '<button class="button_a" onclick="changeImage(',$row['photo_a'],');"></button>';

$row['photo_a'] is the file flower.jpg

which is a code to create the button and call the function changeImage

this only gives me the error ReferenceError: flower is not defined

i believe that this is becuase this is not noticeded as a string, but how can i fix this?