How does php parse variables when echo outputs?

For example, the above code is not valid and cannot be obtained as expected. after checking the code, I think it is a variable & quot; $ xxx & quot; the first line has been parsed, but it is not defined in the first line & quot; $ xxx & quot, so if you don't get the value, how can you let the variable parse this & quot; $ xxx & quot; variable in the output to get this & quot; abc666abc & quot? Online and so on. if you want to pass by, don't give me some advice. thank you! Php

For example:
$ A = "abc {$ xxx} abc ";
$ Xxx = "666 ";
Echo $;
?>
The above code is invalid and cannot get the expected "abc666abc"
After checking the code, the variable "$ xxx" is parsed in the first line,
But in the first line "$ xxx" is not defined, so the value is not obtained.
How can I parse the "$ xxx" variable in the output and get the "abc666abc" variable?

Online and so on. if you want to pass by, don't give me some advice. thank you!