GH-13661: implements gmp_prevprime.

Sets the GMP instance to the greatest prime < than the argument
provided (or left the instance unmodified).

ext/gmp/config.m4

@@ -41,4 +41,5 @@ if test "$PHP_GMP" != "no"; then
   PHP_SUBST([GMP_SHARED_LIBADD])
   AC_DEFINE([HAVE_GMP], [1],
     [Define to 1 if the PHP extension 'gmp' is available.])
+  AC_CHECK_FUNCS([__gmpz_prevprime])
 fi

ext/gmp/gmp.c

@@ -2080,6 +2080,31 @@ ZEND_FUNCTION(gmp_scan1)
 }
 /* }}} */
 
+#ifdef HAVE___GMPZ_PREVPRIME
+ZEND_FUNCTION(gmp_prevprime)
+{
+	zval *a_arg;
+	mpz_ptr gmpnum_a, gmpnum_result;
+	gmp_temp_t temp_a;
+
+	if (zend_parse_parameters(ZEND_NUM_ARGS(), "z", &a_arg) == FAILURE){
+		RETURN_THROWS();
+	}
+
+	FETCH_GMP_ZVAL(gmpnum_a, a_arg, temp_a, 1);
+	INIT_GMP_RETVAL(gmpnum_result);
+
+	/*
+	 * If it leads to a prime number the resulting
+	 * GMP instance will reflect the change anyway ;
+	 * on certainty (returning 2) or probability (1).
+	 */
+	(void)mpz_prevprime(gmpnum_result, gmpnum_a);
+
+	FREE_GMP_TEMP(temp_a);
+}
+#endif
+
 ZEND_METHOD(GMP, __construct)
 {
 	zend_string *arg_str = NULL;

ext/gmp/gmp.stub.php

@@ -184,3 +184,7 @@ function gmp_hamdist(GMP|int|string $num1, GMP|int|string $num2): int {}
 function gmp_nextprime(GMP|int|string $num): GMP {}
 
 function gmp_binomial(GMP|int|string $n, int $k): GMP {}
+
+#ifdef HAVE___GMPZ_PREVPRIME
+function gmp_prevprime(GMP|int|string $num): GMP {}
+#endif

ext/gmp/tests/gh13661.phpt

@@ -0,0 +1,37 @@
+--TEST--
+gmp_prevprime()
+--EXTENSIONS--
+gmp
+--SKIPIF--
+<?php if (!function_exists("gmp_prevprime")) die("skip no gmp_prevprime"); ?>
+--FILE--
+<?php
+
+var_dump(gmp_prevprime(-1));
+var_dump(gmp_prevprime(1));
+var_dump(gmp_prevprime(3));
+var_dump(gmp_prevprime(gmp_init("3")));
+var_dump(gmp_prevprime(gmp_init(PHP_INT_MAX)));
+
+?>
+--EXPECTF--
+object(GMP)#1 (1) {
+  ["num"]=>
+  string(1) "0"
+}
+object(GMP)#1 (1) {
+  ["num"]=>
+  string(1) "0"
+}
+object(GMP)#1 (1) {
+  ["num"]=>
+  string(1) "2"
+}
+object(GMP)#1 (1) {
+  ["num"]=>
+  string(1) "2"
+}
+object(GMP)#1 (1) {
+  ["num"]=>
+  string(19) "%d"
+}