Searching for a email or word inside a text string

$main_string_new="a@gmail.com,yug@gmail.com,a@gmail.com";

if(stristr($main_string_new,"a@gmail.com,")){

echo "String contains a Email id 'a@gmail.com' ";
}
else {
echo "String does not contains a email id 'a@gmail.com' ";
}