A simple example of how to filter a custom value into an applied filter.

freelancer-add-filter-example.php

/*
 * This is the cleanest way to do this, using an anonymous function,
 * but such a function requires PHP 5.3 or higher.
 */
add_filter( 'theme_read_more_text', function() { return 'Read more ->'; } );

/*
 * This is the same anonymous function example,
 * but formatted differently for those who might
 * the first example a bit confusing.
 */
add_filter( 'theme_read_more_text', function() {
  return 'Read more ->';
} );

/*
 * This example is the more "traditional approach
 * using a named function for pre-5.3 PHP compatibility.
 */
add_filter( 'theme_read_more_text', 'my_custom_read_more_text' );
function my_custom_read_more_text() {
  return 'Read more ->';
}