An error occurred while operating mysql syntax in php.

{Code ...} why can't I use the & #039; {$ username} & #039; Method to query mysql Databases in PHP? variables can be parsed using double quotation marks in PHP syntax, why do I encounter errors when I use the first one?

1. Error query statement: $ SQL = 'select * from sh_admin where username = $ username and password = $ password limit 1'; 2. correct query statement: $ SQL = "select * from sh_admin where username = '{$ username}' and password = '{$ password}' limit 1 ";

Why can't I use the '{$ username}' method to query mysql Databases in PHP? The variable can be parsed using double quotation marks in PHP syntax. Why can't I use the first one?

Reply content:

1. Error query statement: $ SQL = 'select * from sh_admin where username = $ username and password = $ password limit 1'; 2. correct query statement: $ SQL = "select * from sh_admin where username = '{$ username}' and password = '{$ password}' limit 1 ";

Why can't I use the '{$ username}' method to query mysql Databases in PHP? The variable can be parsed using double quotation marks in PHP syntax. Why can't I use the first one?

1. Variables $ username and $ password are not parsed in single quotes.
2. The values following the username and password fields must be enclosed in quotation marks.

$sql='select * from sh_admin where username="'.$username.'" and password="'.$password.'" limit 1';

Your first SQL statement uses single quotes, so an error occurs.
Yes

$sql="select * from sh_admin where username=$username and password=$password limit 1";

No problem.

Agree that the string is enclosed in quotation marks.

After debugging$sqlOutput it at a glance!
The variables in the first single quotes are not parsed, And the strings in the SQL statement are to be caused.