Expanding on the suggestion to use the ReflectionFunction, you could use something like this:
$func = new ReflectionFunction('myfunction');
$filename = $func->getFileName();
$start_line = $func->getStartLine() - 1; // it's actually - 1, otherwise you wont get the function() block
$end_line = $func->getEndLine();
$length = $end_line - $start_line;
$source = file($filename);
$body = implode("", array_slice($source, $start_line, $length));
print_r($body);
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