w3schools - How can I get at the matches when using preg_replace in PHP?

preg_replace_all (3)

From the preg_replace documentation on php.net:

replacement may contain references of the form \n or (since PHP 4.0.4) $n, with the latter form being the preferred one. Every such reference will be replaced by the text captured by the n'th parenthesized pattern.

See Flubba's example.

I am trying to grab the capital letters of a couple of words and wrap them in span tags. I am using preg_replace for extract and wrapping purposes, but it's not outputting anything.

preg_replace("/[A-Z]/", "<span class=\"initial\">$1</span>", $str)

Use parentheses around your desired capture.

\0 will also match the entire matched expression without doing an explicit capture using parenthesis.

preg_replace("/[A-Z]/", "<span class=\"initial\">\\0</span>", $str)

As always, you can go to php.net/preg_replace or php.net/<whatever search term> to search the documentation quickly. Quoth the documentation:

\0 or $0 refers to the text matched by the whole pattern.