PHP example of using class constants for well defined and easily maintained options.

MusicSearch.php

<?php

class MusicSearch {

    // -----------------------------------------------------------------------------------------------------------------
    // CONSTANTS
    // -----------------------------------------------------------------------------------------------------------------

    // categories options
    const CATEGORY_ALBUM = 1;
    const CATEGORY_SONG  = 2;

    // search fields options
    const SEARCH_FIELD_ARTIST = 1;
    const SEARCH_FIELD_TITLE  = 2;

    // -----------------------------------------------------------------------------------------------------------------
    // PROPERTIES
    // -----------------------------------------------------------------------------------------------------------------

    private $tables = Array(
        self::CATEGORY_ALBUM => 'album',
        self::CATEGORY_SONG  => 'song',
    );

    private $fields = Array(
        self::SEARCH_FIELD_ARTIST => 'artist',
        self::SEARCH_FIELD_TITLE  => 'title',
    );

    // -----------------------------------------------------------------------------------------------------------------
    // METHODS
    // -----------------------------------------------------------------------------------------------------------------

    public function search ($category, $field, $term) {

        // obviously normally you use escaping strategy like binding
        $searchSql = 'SELECT * FROM `'.$this->tables[$category].'` WHERE `'.$this->fields[$field].'` = "'.$term.'"';

        return $searchSql;

    }

}

// ---------------------------------------------------------------------------------------------------------------------
// EXAPLE USAGE
// ---------------------------------------------------------------------------------------------------------------------

$ms = new MusicSearch();

// Hopefully find Michael Jackson's Thriller album
echo 'Example #1: '.$ms->search(MusicSearch::CATEGORY_ALBUM, MusicSearch::SEARCH_FIELD_TITLE, 'Thriller')."\n\r";

// Hopefully find Soulja Boy's Crank That song
echo 'Example #2: '.$ms->search(MusicSearch::CATEGORY_SONG, MusicSearch::SEARCH_FIELD_TITLE, 'Crank That')."\n\r";

// Hopefully find Paul Simon's albums
echo 'Example #3: '.$ms->search(MusicSearch::CATEGORY_ALBUM, MusicSearch::SEARCH_FIELD_ARTIST, 'Paul Simon')."\n\r";

// Hopefully find Swollen Members' songs
echo 'Example #4: '.$ms->search(MusicSearch::CATEGORY_SONG, MusicSearch::SEARCH_FIELD_ARTIST, 'Swollen Members')."\n\r";

output.txt

~ » php -f MusicSearch.php
Example #1: SELECT * FROM `album` WHERE `title` = "Thriller"
Example #2: SELECT * FROM `song` WHERE `title` = "Crank That"
Example #3: SELECT * FROM `album` WHERE `artist` = "Paul Simon"
Example #4: SELECT * FROM `song` WHERE `artist` = "Swollen Members"