Gettext_Translations::set_header( string $header, string $value )

Source

File: wp-includes/pomo/translations.php

function set_header( $header, $value ) {
			parent::set_header( $header, $value );
			if ( 'Plural-Forms' === $header ) {
				list( $nplurals, $expression )     = $this->nplurals_and_expression_from_header( $this->get_header( 'Plural-Forms' ) );
				$this->_nplurals                   = $nplurals;
				$this->_gettext_select_plural_form = $this->make_plural_form_function( $nplurals, $expression );
			}
		}