The gmp_nextprime() is an inbuilt function in PHP which is used to calculate the prime number just greater than a given GMP number(GNU Multiple Precision : For large numbers).
Syntax:
gmp_nextprime($num)
Parameters: This function accepts a GMP number $num as a mandatory parameter as shown in the above syntax whose next prime we want to find. This parameter can be a GMP object in PHP version 5.6 and later, or we are also allowed to pass a numeric string provided that it is possible to convert that string to a number.
Return Value: This function returns a GMP number which is the prime number just greater than the GMP number passed to it as a parameter.
Examples:
Input : gmp_nextprime("15")
Output : 17
Input : gmp_nextprime("21")
Output : 23
Below programs illustrate the gmp_nextprime() function in PHP :
Program 1: Program to calculate the prime number just greater than a GMP number when numeric string as GMP number is passed as an argument.
<?php // PHP program to calculate the prime number greater // than that of a GMP number passed as arguments    // strings as GMP numbers $num = "135";   // gives the prime number just greater // than 135 as a GMP number $res = gmp_nextprime($num);   echo $res;   ?> |
Output:
137
Program 2: Program to calculate the prime number greater than that of a GMP number when GMP number is passed as an argument.
<?php // PHP program to calculate the prime  // number greater than that of a GMP number   // creating GMP numbers using gmp_init() $num = gmp_init(1000);   // gives the prime number just greater // than 1000 as a GMP number $res = gmp_nextprime($num);   echo $res;   ?> |
Output:
1009
Reference:
http://php.net/manual/en/function.gmp-nextprime.php
