What is the special meaning of $ in php? _ What are the special meanings?

Some problems were found during local debugging. Is this the code output result $ playing a special role?

Problems found during local debugging

This is the code

Output result

$ Has played a special role?

Reply content:

Problems found during local debugging

This is the code

Output result

$ Has played a special role?

Echo "$ _"; // in "", variables are identified, and $ _ is a variable. Echo '$ _'; // in '', the content is completely output, that is, the content is printed whatever you write, regardless of the variability.