Description:
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From manual page: http://www.php.net/function.preg-replace
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"When working with a replacement pattern where a backreference is immediately followed by another number (i.e.: placing a literal number immediately after a matched pattern), you cannot use the familiar \\1 notation for your backreference. \\11, for example, would confuse preg_replace() since it does not know whether you want the \\1 backreference followed by a literal 1, or the \\11 backreference followed by nothing. In this case the solution is to use \${1}1. This creates an isolated $1 backreference, leaving the 1 as a literal."

The second-last sentence is confusing since this appears to contradict 2 other parts of the page: the paragraph above which says "replacement may contain references of the form \\n or (since PHP 4.0.4) $n" (no \$), and Example #1 which uses single-quoted string so doesn't require \$.

It would be less confusing if all parts of the page used the same style when describing $-delimited backreferences. #47797 also suggested drawing more attention to the escaping requirements, which could be helpful.