How to replace different newline styles in PHP the smartest way?

by 
$string = preg_replace('~\R~u', "\r\n", $string);

If you don’t want to replace all Unicode newlines but only CRLF style ones, use:

$string = preg_replace('~(*BSR_ANYCRLF)\R~', "\r\n", $string);

\R matches these newlines, u is a modifier to treat the input string as UTF-8.

From the PCRE docs:

What \R matches

By default, the sequence \R in a pattern matches any Unicode newline
sequence, whatever has been selected as the line ending sequence. If
you specify

     --enable-bsr-anycrlf

the default is changed so that \R matches only CR, LF, or CRLF. Whatever is selected when PCRE is built can be overridden when the library
functions are called.

and

Newline sequences

Outside a character class, by default, the escape sequence \R matches
any Unicode newline sequence. In non-UTF-8 mode \R is equivalent to the
following:

    (?>\r\n|\n|\x0b|\f|\r|\x85)

This is an example of an “atomic group”, details of which are given
below. This particular group matches either the two-character sequence
CR followed by LF, or one of the single characters LF (linefeed,
U+000A), VT (vertical tab, U+000B), FF (formfeed, U+000C), CR (carriage
return, U+000D), or NEL (next line, U+0085). The two-character sequence
is treated as a single unit that cannot be split.

In UTF-8 mode, two additional characters whose codepoints are greater
than 255 are added: LS (line separator, U+2028) and PS (paragraph separator, U+2029). Unicode character property support is not needed for
these characters to be recognized.

It is possible to restrict \R to match only CR, LF, or CRLF (instead of
the complete set of Unicode line endings) by setting the option
PCRE_BSR_ANYCRLF either at compile time or when the pattern is matched.
(BSR is an abbrevation for “backslash R”.) This can be made the default
when PCRE is built; if this is the case, the other behaviour can be
requested via the PCRE_BSR_UNICODE option. It is also possible to
specify these settings by starting a pattern string with one of the
following sequences:

    (*BSR_ANYCRLF)   CR, LF, or CRLF only
    (*BSR_UNICODE)   any Unicode newline sequence

These override the default and the options given to pcre_compile() or
pcre_compile2(), but they can be overridden by options given to
pcre_exec() or pcre_dfa_exec(). Note that these special settings, which
are not Perl-compatible, are recognized only at the very start of a
pattern, and that they must be in upper case. If more than one of them
is present, the last one is used. They can be combined with a change of
newline convention; for example, a pattern can start with:

    (*ANY)(*BSR_ANYCRLF)

They can also be combined with the (*UTF8) or (*UCP) special sequences.
Inside a character class, \R is treated as an unrecognized escape
sequence, and so matches the letter “R” by default, but causes an error
if PCRE_EXTRA is set.

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