Programmatically implement SearchWP synonyms (you do not need the Term Synonyms extension)

gistfile1.php

<?php

add_filter( 'searchwp_term_in', 'my_find_synonyms', 10, 2 );
add_filter( 'searchwp_and_logic', '__return_false' );

function my_find_synonyms( $term, $engine ) {
	global $searchwp;

	if( ! class_exists( 'SearchWP' ) || version_compare( $searchwp->version, '2.0.3', '<' ) ) {
		return $term;
	}

	$synonyms = array(
		array(
			'term'     => 'ny',
			'synonyms' => 'new york',
		),
		array(
			'term'     => 'nj',
			'synonyms' => 'new jersey',
		)
	);

	// we expect $term to be an array
	if( is_string( $term ) ) {
		$term = array( $term );
	}

	if( is_array( $term ) && is_array( $synonyms ) && !empty( $synonyms ) ) {
		foreach( $synonyms as $synonym ) {
			if( in_array( $synonym['term'], $term ) ) {

				// there is a match, handle it

				// break out where applicable
				if( is_array( $synonym['synonyms'] ) && ! empty( $synonym['synonyms'] ) ) {
					foreach( $synonym['synonyms'] as $maybe_synonym ) {
						if( false !== strpos( $maybe_synonym, ' ' ) ) {
							$maybe_synonym = explode( ' ', $maybe_synonym );
							$synonym['synonyms'] = array_merge( $synonym['synonyms'], $maybe_synonym );
						}
					}
				}

				// merge everything together
				$term = array_merge( $term, $synonym['synonyms'] );
			}
		}
	}

	$term = array_values( array_unique( $term ) );
	$term = array_map( 'sanitize_text_field', $term );

	return $term;
}