as you know that if , \(a\geq b\) ,then it is not necessary that \(\phi(a)\geq\phi(b)\) . Since \(\phi\) not an increasing function.
But there are some cases in which above inequality holds true.
When either of \(a\) and \(b\) , suppose lets take \(a\) as an arbitary prime , then \(b=a+1\) or \(b=a-1\). Same follows if \(b\) is prime.
When \(a\) and \(b\) are both primes and , \(a>b\)