Kepp the following Quote in mind:

If eval() is the answer, you're almost certainly asking the
wrong question. -- Rasmus Lerdorf, BDFL of PHP

Inception with eval()

<pre>
Inception Start:
<?php
eval("echo 'Inception lvl 1...\n'; eval('echo \"Inception lvl 2...\n\"; eval(\"echo \'Inception lvl 3...\n\'; eval(\'echo \\\"Limbo!\\\";\');\");');");
?>

At least in PHP 7.1+, eval() terminates the script if the evaluated code generate a fatal error. For example:
<?php
@eval('$content = (100 - );');
?>

(Even if it is in the man, I'm note sure it acted like this in 5.6, but whatever)
To catch it, I had to do:
<?php
try {
    eval('$content = (100 - );');
} catch (Throwable $t) {
    $content = null;
}
?>

This is the only way I found to catch the error and hide the fact there was one.

If you want to allow math input and make sure that the input is proper mathematics and not some hacking code, you can try this:

<?php

$test = '2+3*pi';

// Remove whitespaces
$test = preg_replace('/\s+/', '', $test);

$number = '(?:\d+(?:[,.]\d+)?|pi|π)'; // What is a number
$functions = '(?:sinh?|cosh?|tanh?|abs|acosh?|asinh?|atanh?|exp|log10|deg2rad|rad2deg|sqrt|ceil|floor|round)'; // Allowed PHP functions
$operators = '[+\/*\^%-]'; // Allowed math operators
$regexp = '/^(('.$number.'|'.$functions.'\s*\((?1)+\)|\((?1)+\))(?:'.$operators.'(?2))?)+$/'; // Final regexp, heavily using recursive patterns

if (preg_match($regexp, $q))
{
    $test = preg_replace('!pi|π!', 'pi()', $test); // Replace pi with pi function
    eval('$result = '.$test.';');
}
else
{
    $result = false;
}

?>

I can't guarantee you absolutely that this will block every possible malicious code nor that it will block malformed code, but that's better than the matheval function below which will allow malformed code like '2+2+' which will throw an error.

It should be noted that imported namespaces are not available in eval.

The following code

<?php
    eval( '?> foo <?php' );
?>

does not throw any error, but prints the opening tag.
Adding a space after the open tag fixes it:

<?php
    eval( '?> foo <?php ' );
?>

imo, this is a better eval replacement: 

<?php
function betterEval($code) {
    $tmp = tmpfile ();
    $tmpf = stream_get_meta_data ( $tmp );
    $tmpf = $tmpf ['uri'];
    fwrite ( $tmp, $code );
    $ret = include ($tmpf);
    fclose ( $tmp );
    return $ret;
}
?>

- why? betterEval follows normal php opening and closing tag conventions, there's no need to strip `<?php?>` from the source.  and it always throws a ParseError if there was a parse error, instead of returning false (note: this was fixed for normal eval() in php 7.0). - and there's also something about exception backtraces