Description:
------------
in php-7.2.15 source 'Zend/zend_constants.c':

ZEND_API int zend_register_constant(zend_constant *c)
{
...
#479  char *slash = strrchr(ZSTR_VAL(c->name), '\\');
...
#482  zend_str_tolower(ZSTR_VAL(lowercase_name), slash - ZSTR_VAL(c->name));

-----------
 constant name is changed to "foo\\BAR"

but in the same file 
#243   ZEND_API zval *zend_get_constant_str(const char *name, size_t name_len)
...
#250  zend_str_tolower_copy(lcname, name, name_len);

---------
use "Foo\\BAR" to find first,and try "foo\\bar" again.
both faied.
  

Test script:
---------------
1. REGISTER_NS_STRING_CONSTANT("Foo", "BAR", "Test", CONST_CS | CONST_PERSISTENT);
2. zval *c = zend_get_constant_str(ZEND_STRL("Foo\\BAR"));

it returns NULL;