How can I write a path for a srcset in php

Hello dear colleagues, I have a simple code that prints a string with an echo but it does not print the string complete here the code

<img class="columna" srcset=<?php print addslashes( "../img/{$producto- 
>imagen}500.jpg 300w,../img/{$producto->imagen}500.jpg 
600w,../img/{$producto->imagen}1000.jpg 1000w,../img/{$producto- 
>imagen}1500.jpg 1500w");?> src=<?php echo"../img/{$producto- 
>imagen}1500.jpg"?> alt=<?php echo $producto->descripcion ?>>
        </section><?php endforeach; ?>