(ShareCast News) - WPP celebrated its 30th birthday in style in 2015, with another record year of results despite strong currency headwinds.Reported billings at the advertising and public relations firm were up 3.1% to £47.63bn in its preliminary results on Friday, though the revenue results were mixed through its various markets.Pound sterling revenue was up 6.1% to £12.235bn, and euro revenue rose 17.8% to €16.874bn. Dollar revenue was down 1.4% at $18.693bn, and yen revenue dropped 12.4% to JPY 2.264trn.Constant currency revenue growth was 7.5%, however, with like-for-like revenue up 5.3%. WPP's constant currency net sales were up 5.8%, and like-for-like net sales rose 3.3%.The company reported a net sales margin of 16.9%, up 0.2 points against the prior year, and 0.4 points on a constant currency basis. That was ahead of the full-year target of 0.3 points in constant currency.Headline EBITDA crossed the £2bn threshold for the first time, at £2.002bn, and headline profit before tax rose 7.3% to £1.62bn - an 11.2% rise in constant currencies.Reported profit before tax rose 2.8%, or 7.3% in constant currencies, to £1.49bn.WPP's headline diluted earnings per share were up 10.2% to 93.6p, or up 13.3% in constant currencies. Its return on equity during the year reached 16.3%, up 1.3 percentage points on 2014, and against a weighted average cost of capital of 6.7% in 2015.Net debt stood at £3.211bn on 31 December 2015, an increase of £936m on a year prior.Net new business totalled £5.557bn during the period, with the group first overall in new business league tables for the fourth year in a row and GroupM topping both the RECMA media tsunami net new business and retention tables.The FTSE 100 firm confirmed a 17% rise in total dividends for the year, to 17p per share. That represented a payout ratio of 47.7%, against 45% last year.WPP's board said the company had made an above-budget start to 2016, with January like-for-like revenue up 4.2% and net sales up 2.3% against strong comparatives.