Single double quote problem when alert pops up in PHP

The problem of single and double quotation marks in PHP code is a very important problem, which can cause a lot of trouble when used improperly. First record the trouble of writing alert today. When I do login, I want to pop up a prompt to say that login is successful. Does not appear when the alert statement is written. In fact, the single double quotation marks are wrong. The correct statement should be:

I tested as long as a single double quote is not used at the same time, otherwise it will not be displayed. I should then summarize the use rules for single and double quotes.

Additionally, when pop-ups and jumps appear in a piece of code, alert does not appear and jumps directly to the targeted page. If you want alert to appear, you have to write alert in the front of the location page.

Single double quote problem when alert pops up in PHP