It seems gmp_nextprime on negative numbers always returns 2.
(PHP 5 >= 5.2.0, PHP 7)
gmp_nextprime — Find next prime number
Find next prime number
Return the next prime number greater than
as a GMP number.
Example #1 gmp_nextprime() example
$prime1 = gmp_nextprime(10); // next prime number greater than 10
$prime2 = gmp_nextprime(-1000); // next prime number greater than -1000
echo gmp_strval($prime1) . "\n";
echo gmp_strval($prime2) . "\n";
The above example will output:
This function uses a probabilistic algorithm to identify primes and chances to get a composite number are extremely small.
Negative numbers N always have at least four factors: N*1=N and abs(N)*-1=N. Therefore, even if "natural number" weren't in the definition of a prime (which it is), no negative number would ever qualify. Therefore, the next-largest prime number to any negative number is always 2.