docs(sompose): Replace echo with variable assignment to prevent inval…

…id JSON output (#1345)

doc/producers/composing.md

@@ -40,7 +40,7 @@ Note that you can always easily tap into the chain and e.g. use xdebug to debug
   $builder->compose(
       $builder->tap($builder->callback(function ($parent, $args) {
           // YOU CAN SET A XDEBUG BREAKPOINT IN THESE CALLBACKS TO CHECK THE VALUES.
-          echo("Compose step 0.");
+          $compose_step = 0;
       })),
       // Load the file object from the field.
       $builder->produce('property_path')
@@ -49,22 +49,22 @@ Note that you can always easily tap into the chain and e.g. use xdebug to debug
         ->map('path', $builder->fromValue('YOUR_FIELD_NAME.YOUR_FIELD_PROPERTY')),
       $builder->tap($builder->callback(function ($parent, $args) {
           // YOU CAN SET A XDEBUG BREAKPOINT IN THESE CALLBACKS TO CHECK THE VALUES.
-          echo("Compose step 1.");
+          $compose_step = 1;
       })),
       // Load the image style derivative of the file.
       $builder->produce('image_derivative')
         ->map('entity', $builder->fromParent())
         ->map('style', $builder->fromValue('YOUR_IMAGE_STYLE')),
       $builder->tap($builder->callback(function ($parent, $args) {
           // YOU CAN SET A XDEBUG BREAKPOINT IN THESE CALLBACKS TO CHECK THE VALUES.
-          echo("Compose step 2.");
+          $compose_step = 2;
       })),
       // Retrieve the url of the generated image.
       $builder->produce('image_style_url')
         ->map('derivative', $builder->fromParent()),
       $builder->tap($builder->callback(function ($parent, $args) {
           // YOU CAN SET A XDEBUG BREAKPOINT IN THESE CALLBACKS TO CHECK THE VALUES.
-          echo("Compose step 3.");
+          $compose_step = 3;
       }))
     )
   );