PHP operation MySQL Syntax format error

1、错误查询语句：$sql='select * from sh_admin where username=$username and password=$password limit 1';2、正确查询语句：$sql = "select * from sh_admin where username = '{$username}' and password = '{$password}' limit 1";

When PHP operation MySQL database Why must use ' {$username} ' way to query, PHP syntax double quotes can parse variables Ah, why do I use the first kind of error?

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1、错误查询语句：$sql='select * from sh_admin where username=$username and password=$password limit 1';2、正确查询语句：$sql = "select * from sh_admin where username = '{$username}' and password = '{$password}' limit 1";

When PHP operation MySQL database Why must use ' {$username} ' way to query, PHP syntax double quotes can parse variables Ah, why do I use the first kind of error?

1, variables $username and $password in single quotes is not resolved
2. The values after the Character field username and password need to be quoted.

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

Your first SQL uses single quotes, so it's a mistake.
is to use

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

There's no problem.

Agree upstairs, the string is to be quoted.

Debugging will $sql output a bit, you can see at a glance!
The variable in the first single quotation mark is not parsed, and the string inside the SQL is to be caused.

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