Reading time calculation from one of my WordPress plugins. MIT licensed. Use as you see fit.

reading_time.php

<?php

  /**
	 * Calculate the reading time for a piece of content
	 *
	 * @param string  $content
	 * @param integer $wpm Words Per Minute reading speed
	 *
	 * @return array minutes => integer, seconds => integer
	 */
	private static function calc_reading_time( $content, $wpm = 250 ) {

		$wpm = absint( $wpm );

		$word            = str_word_count( strip_tags( $content ) );
		$time            = array();
		$time['minutes'] = floor( $word / $wpm );
		$time['seconds'] = floor( $word % $wpm / ( $wpm / 60 ) );
		return $time;

	}