Rory McIlroy came into the PGA Championship hot, having backed his epic Open Championship victory at Hoylake with a two-shot triumph at the WGC-Bridgestone Invitational in his next start, climbing back into the number one sport in the Official World Ranking in the process.  Thus he arrived at Louisville's Valhalla Golf Club as the strongest Major championship favorite since Tiger Woods - and seemingly very much at home with the expectations.  McIlroy's performance in Thursday's opening round did little to suggest the the oddsmakers were wrong, for playing over a rain-softened Valhalla layout that would yield low scores all week, he posted a five-under-par 66 that left him one shot off the lead held by England's Lee Westwood and a pair of Americans, Kevin Chappell and Ryan Palmer.  What made McIlroy's round notable, however, was that it might have been quite a bit lower, for after turning in three-under-par 32, he hooked his second at the par-5 10th out-of-bounds en route to a double-bogey, then followed that with a bogey at the 210-yard 11th.  But where many a golfer might have lost their momentum entirely after falling back to even, McIlroy instead responded with four straight birdies at holes 12-15, then added one more at the par-5 closer to push himself resiliently back into the mix.  Friday saw McIlroy play in the morning and it was then that he began taking control of the tournament, piecing together an up-and-down 67 that included four birdies and an eagle (at his ninth hole, the par-5 18th) and finishing with a tournament-leading nine-under-par 133 total.  His margin was only one, however, for hot on his heels were the ever-competitive Jim Furyk (68) and three-time Major championship runner-up Jason Day, who posted the day's low round (65) by logging five birdies, plus an eagle at the 597-yard split-fairway 7th.  Following more morning rain, Saturday would prove to be the lowest scoring round in PGA Championship history, ultimately producing a 54-leaderboard that was both deep and star-studded.  McIlroy may not have loved his round of 67 but on a day when a number of stars might well have shot past him, he gamely rallied late in the back nine to birdie the 15th, 16th and 18th and maintain a one-shot lead.  Now his closest pursuer was Austrian upstart Bernd Wiesberger, who carded a flawless 65 that included birdies on the final three holes, and moved him into air far more rarified than anything he'd experienced on the European circuit.  Two back was Rickie Fowler (playing with great confidence as he sailed around with a bogey-free 67) while three behind were Phil Mickelson (67) and Day, who played uneven golf en route to a 69.  Sunday morning witnessed still more rain, with play delayed nearly 90 minutes and questions emerging as to whether the round could be completed before darkness.  As things ultimately unfolded, players like Ernie Els, Jimmy Walker and Furyk all made some early noise, but the real challengers proved to be Mickelson, Fowler and Henrik Stenson.  Trying gamely to salvage an otherwise disappointing year, Mickelson charged to an outgoing 31 and by the 12th tee stood 15 under par.  Fowler, meanwhile, bogeyed the 2nd but bounced back with four birdies in five holes and, with one more at the par-5 10th, also stood on 15 under.  Stenson, for his part, turned in 30 and, with a birdie at the island green 13th, joined them at -15.  McIlroy, meanwhile, had stumbled to an outgoing 36, leaving him, quite surprisingly, three off the pace.  But at a moment when almost any sane golfing mind might well have concluded that his run of form was cooling, McIlroy instead mounted an epic charge, ripping a 281-yard 3 wood to seven feet en route to eagling the 10th, then adding a birdie at the 13th which, combined with each of his opponents carding one untimely bogey, saw him nose improbably back ahead.  A wayward drive at the 17th threatened to pull him back, but a 9 iron from a fairway bunker dropped 11 feet from the hole, with the ensuing birdie putt extending the lead to two.  Yet there was still some drama left.  With darkness rapidly setting in, Mickelson and Fowler followed standard Tour etiquette in allowing McIlroy and Wiesberger to drive off quickly behind them at the last, guaranteeing all four players the option to finish even if the horn sounded.  McIlroy's tee ball then narrowly missed right-side water before things became confusing when PGA of America officials then instructed Mickelson and Fowler to stand aside while the final pair hit their second shots - an odd suggestion in any circumstance.  In the end, Mickelson narrowly missed holing a long pitch to tie, Fowler missed his long eagle putt as well, and when McIlroy took two putts from 33 feet for his five, the championship was his.  Fowler, for his part, became the only player in history to finish among the top five in all four of a season's Major championships without winning one.  McIlroy, meanwhile, deeply solidified his hold on world number one status, and also became only the fourth player in the last century (joining Jones, Nicklaus and Woods) to win four Majors by age 25.  Most importantly, however, he gave great legitimacy to the notion that he may now, in fact, be a dominant player with the ability to win on a scale similar to Woods and Nicklaus.