After making some extensions, I found at the doc that 
zend_get_parameters_ex() was supposed to be depreciated. As the doc is saying, I passed my script to zend_parse_parameters(). It works fine with zval*, long, double etc... But It looks it segfaults with a zval**.

ZEND_FUNCTION(calculhmac)
{
	zval **clent, **data;
	char   *result;
	char   *iclent, *idata;
	if (zend_parse_parameters(2, "zz", &clent, &data) == FAILURE) {
		WRONG_PARAM_COUNT;
	}

	convert_to_string_ex(clent);
	convert_to_string_ex(data);

	iclent = Z_STRVAL_PP(clent);
	idata = Z_STRVAL_PP(data);

	result = sp_CalculHmac(iclent, idata);

	RETURN_STRING(result, 1);

}

Note that it works fine with get and that parse is supposed to replace get...

Here is the gdb report:
Program received signal SIGSEGV, Segmentation fault.
0x4011baeb in strlen (str=0x0) at ../sysdeps/i386/strlen.c:28
28      ../sysdeps/i386/strlen.c: No such file or directory.
        in ../sysdeps/i386/strlen.c

Thanks to verify that before closing.