Wordpress: How to save custom taxonomy terms in json when a post saved.

functions.php

// Add this code to bottom line of your active theme functions.php file.

function export_custom_terms_in_json () {
  
  // get all terms of custom_taxonomy
	$CustomTaxTerms = get_terms( 'custom_taxonomy',  array( 'orderby'=>'count', 'hide_empty' => 0 ) );

	if( ! is_wp_error( $CustomTaxTerms ) ) {
    
    // encode term list
		$data = json_encode( $CustomTaxTerms );
    
    // define the directory where json file will be saved.
		$folder = get_template_directory();

    // specify the name of the file to save.
		$file_name = 'MyTerms.json';

		$fileUrl = $folder . '/' . $file_name;

		file_put_contents( $fileUrl, $data );
	}
}
add_action( 'save_post', 'export_custom_terms_in_json' );