Properly pluralize string.

pluralize-string.php

<?php
/**
 * Define irregular plurals
 *
 * @return array Array of irregular plurals
 */
function zzap_irregular_plurals() {

	$irregulars = array(
		'man'           => 'men',
		'woman'         => 'women',
		'fungus'        => 'fungi',
		'thief'         => 'thieves',
		'medium'        => 'media',
		'person'        => 'people',
		'echo'          => 'echoes',
		'hero'          => 'heroes',
		'potato'        => 'potatoes',
		'veto'          => 'vetoes',
		'auto'          => 'autos',
		'memo'          => 'memos',
		'pimento'       => 'pimentos',
		'pro'           => 'pros',
		'knife'         => 'knives',
		'leaf'          => 'leaves',
		'bus'           => 'busses',
		'child'         => 'children',
		'quiz'          => 'quizzes',
		# words whose singular and plural forms are the same
		'equipment'     => 'equipment',
		'fish'          => 'fish',
		'information'   => 'information',
		'money'         => 'money',
		'moose'         => 'moose',
		'news'          => 'news',
		'rice'          => 'rice',
		'series'        => 'series',
		'sheep'         => 'sheep',
		'species'       => 'species'
	);
	// filter if needed
	return apply_filters( 'zzap_irregular_plurals', $irregulars );
}
/**
 * Pluralize string
 *
 * Check if string is irregular and, relevant to its ending,
 * create plural form.
 *
 * @param  string $string Singular that needs to be pluralized
 * @return string         Returns pluralized sting
 */
function zzap_pluralize( $string ) {
	// get array with irregulars
	$irregulars = zzap_irregular_plurals();
	// endings that get 'es' in plural
	$es = array( 's', 'z', 'ch', 'sh', 'x' );
	// get the last letter from string
	$last_letter = substr( $string, -1 );

	// string found in irregulars, apply plural from irregulars array
	if ( array_key_exists( $string, $irregulars ) ) :
		return $irregulars[$string];

	// strings ending with 'y'
	elseif ( $last_letter == 'y' ) :
		// if last two characters are 'ey' replace both with 'ies'
		if ( substr( $string, -2 ) == 'ey' ) :
			return substr_replace( $string, 'ies', -2, 2 );
		// otherwise replace just last one
		else :
			return substr_replace( $string, 'ies', -1, 1 );
		endif; //  substr( $string, -2 ) == 'ey' 
		
	// string's end match 'es' array, add that 'es' 
	elseif ( in_array( substr( $string, -2 ), $es ) || in_array( substr( $string, -1 ), $es ) ) :
		return $string . 'es';

	// any other case
	else :
		return $string . 's';
	
	endif; // array_key_exists( $string, $irregulars ) 
}