PHP statement line breaks, each line is controlled within 80 characters

In code writing, follow the following principles:
a. Try to ensure that the program statement is one sentence per line, and do not make a line of statement too long and cause line breaks;
b. Try not to make a line of code too long, generally within 80 characters;
c. If a line of code is too long, please use a line break like .=;
d. For SQL statement operations that execute databases, try not to write SQL statements within functions, but first define the SQL statement with variables, and then call the defined variables in the function that performs the operation;
Example:

$sql = "SELECT username,password,address,age,postcode FROM test_t ";
$sql .= " WHERE username='aaa'";
$res = mysql_query($sql);