These are Unicode escape sequences that reference characters in the Unicode character set by denoting their code points in hexadecimal.
From the JSON specification:
Any character may be escaped. If the character is in the Basic
Multilingual Plane (U+0000 through U+FFFF), then it may be
represented as a six-character sequence: a reverse solidus, followed
by the lowercase letter u
, followed by four hexadecimal digits that
encode the character's code point. The hexadecimal letters A
though
F
can be upper or lowercase. So, for example, a string containing
only a single reverse solidus character may be represented as
"\u005C
".
Although these characters do not need to be escaped (see unescaped rule), json_encode
does encode any character except those character that are also in US-ASCII (see source of json.c) to avoid encoding issues with US-ASCII-based protocols.
So inside a JSON string, \u0435
references the character at U+0435 that is the CYRILLIC SMALL LETTER IE (е
) and \u0434
references the character at U+0434 that is the CYRILLIC SMALL LETTER DE (д
).