When you set a cookie PHP must add headers to the HTTP Response.
The problem is that when you start printing content, even it that content is just a single white space before the <?php tag, the headers are sent, at which time PHP is unable to add new headers.
So, that means you must set any headers, including cookies, before you send anything else.
If you absolutely must send some content before setting the cookie, you can use
Output buffering to delay the output until you chose to send it.
Somewhat like:
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<?php
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ob_start(); // Start buffering output
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echo "This stuff just had to be sent before the cookie!";
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setcookie(...);
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ob_end_flush(); // Release the output to the response
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?>
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