PHP: putting string and a number with calculation together

In a form I created a loop to show a list of files and next to each file a check box. To give each check biox a unique name, I wanted to add the number of the file to it. So first I did:

<input type="checkbox" name="<?php echo 'upload' . $a-1; ?>">

I made the calculation because I already increased $a before the HTML code came. The result was:

<input type="checkbox" name="-1">

The trick is to put the calculation between brackets, like this:

<input type="checkbox" name="<?php echo 'upload' . ($a-1;) ?>">

And this results in the 'name' being: "upload1", "upload2" etc.

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