PHP MYSQL update - replace whole word, but not part of it

Lets say the field name is bla and the value 123

When I do:

mysqli_query($con, "UPDATE table SET bla = REPLACE (bla, 123, 777)");

123 becomes 777 and it's good

But when I do:

mysqli_query($con, "UPDATE table SET bla = REPLACE (bla, 1, 777)");

I DON'T want 123 to become 77723 but stay 123

How can I do that?