reconstruct/get code of php function

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);

There isn't anything that will give you the actual code of the function. The only thing close to that available is the ReflectionFunction class. For classes you have ReflectionClass that gives you the class members (constants, variables and methods) and their visibility, but still no actual code.

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Workaround (it does involve reading the source file):
Use ReflectionFunction::export to find out the file name and line interval where the function is declared, then read the content from that file on those lines. Use string processing to get what's between the first { and the last }.

Note: The Reflection API is poorly documented. ReflectionFunction::export is deprecated since PHP 7.4

We program through different operating systems, gnu/linux, windows, mac... Due to this, we have different carriage returns in the code, to solve this, I forked the Brandon Horsley's answer and prepare to check different CR and get code from an class's method instead of a function:

$cn = 'class_example';
$method = 'method_example';

$func = new ReflectionMethod($cn, $method);

$f = $func->getFileName();
$start_line = $func->getStartLine() - 1;
$end_line = $func->getEndLine();
$length = $end_line - $start_line;

$source = file($f);
$source = implode('', array_slice($source, 0, count($source)));
// $source = preg_split("/(\n|\r\n|\r)/", $source);
$source = preg_split("/".PHP_EOL."/", $source);

$body = '';
for($i=$start_line; $i<$end_line; $i++)
    $body.="{$source[$i]}\n";

echo $body;