PHP Basics-the difference between double quotes and single quotes

PHP in the previous cognition, the single quotation mark and double quotation marks are not different, if the outside use of single quotation marks, then the inside is double quotation marks, if the outside using double quotation marks, inside the use of single quotation marks, always thought that single quotes, double quotes is the difference, but there is a little difference.

The difference is that the contents of the double-quote string can be interpreted and replaced, and the contents of the single-quote string are always considered ordinary characters.

 
   
The result is output: Hello world! $str
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