Underflow

Underflow happens when a value goes below a limit: out of its range of existence, the behavior of the value is now unknown.

Underflow happens with integers, which are limited from PHP_INT_MIN to PHP_INT_MIN, floats, which are limited from PHP_FLOAT_MIN to PHP_FLOAT_MAX.

Other situations, such as accessing an array element or a string character beyond its first element, defaults to returning null.

<?php

    $a = PHP_INT_MAX;
    $b = (int) ($a +  1);

    echo $a.PHP_EOL;
    echo $b;

?>

Documentation

See also PHP-FPM Underflow RCE and Integer Underflow.

Related : Null, Overflow, RangeException