Inverse behavior to the wpautop() function found in WordPress

unautop-func.php

<?php

/**
 * Replaces paragraph elements with double line-breaks.
 *
 * This is the inverse behavior of the wpautop() function
 * found in WordPress which converts double line-breaks to
 * paragraphs. Handy when you want to undo whatever it did.
 * 
 * @see    wpautop()
 *
 * @param  string $pee
 * @param  bool   $br (optional)
 *
 * @return string
 */
function fjarrett_unautop( $pee, $br = true ) {

	// Match plain <p> tags and their contents (ignore <p> tags with attributes)
	$matches = preg_match_all( '/<(p+)*(?:>(.*)<\/\1>|\s+\/>)/m', $pee, $pees );

	if ( ! $matches ) {

		return $pee;

	}

	$replace = array( "\n" => '', "\r" => '' );

	if ( $br ) {

		$replace['<br>']   = "\r\n";
		$replace['<br/>']  = "\r\n";
		$replace['<br />'] = "\r\n";

	}

	foreach ( $pees[2] as $i => $tinkle ) {

		$replace[ $pees[0][ $i ] ] = $tinkle . "\r\n\r\n";

	}

	return rtrim(
		str_replace(
			array_keys( $replace ),
			array_values( $replace ),
			$pee
		)
	);

}