php-php How do I parse the variable at echo output?

Php

Give me a chestnut:
$a = "abc{$xxx}abc";
$xxx = "666";
echo $a;
?>
The above code is not established, can not get the expected "ABC666ABC"
After examining the code, the individual considers the variable "$xxx" to have been parsed in the first line,
But in the first line "$xxx" is not defined, so the value is not
How can I get this "abc666abc" when the variable is parsed in the output and then the "$xxx" variable?

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