I was going through the text Introduction to Theory of Computation by Michael Sipser. It is written that $R\circ\emptyset$ may not equal to $R$. I thought $R\circ\emptyset=\emptyset$ always. $R$ is any regular expression.Couldnt find any example that contradicts that
Why $R\circ\emptyset$ is not always equal to $\emptyset$
automata;regular expressions
Assuming by $.$ you mean concatention ($\circ$ when made explicit by Sipser) and by $\phi$ you mean $\emptyset$, I'm not sure what the confusion is. Sipser is claiming that sometimes (in almost every case actually), $R\circ\emptyset \neq R$. You are correct in saying that $R\circ\emptyset = \emptyset$, so the only case where $R\circ\emptyset = R$ is when $L(R)=\emptyset$. For any regular expression that generates at least one string (including $\varepsilon$), $L(R) \neq \emptyset$ but $L(R\circ\emptyset) = \emptyset$, so $R\circ\emptyset \neq R$.