Variable Declaration In PHP, variables are defined with the $ sign.$variable = variable value; There are some rules for defining variables. Variables cannot start with a number. Turkish characters cannot be used. Whitespace cannot be used. It is case-sensitive. A declaration like $variable_1 can be made, and any value can be assigned to the variable.$add = (mysql_query("INSERT INTO TableName (Field1, Field2, ...) VALUES ('Value1', 'Value2', ...)");When this variable is executed, it will add data to the database.$write = "Alpay"; The value of the variable is Alpay. When we print this variable to the screen, it will display Alpay. $write = "Alpay"; echo $write; echo "My name is: ".$write; This will output "My name is: Alpay" to the screen.When printing variables, we do not use ' symbols. We use the . symbol to concatenate both normal text and variables simultaneously. . is a concatenation symbol. echo 'My name is: '; echo $write; This will also output the same result, but instead of using two echo commands, we concatenate the two values to be printed with the . symbol. If - Else Structure if (conditions you want to be met) {code to be executed when the condition is met}elseif (checks this condition if the previous condition is not met) {Code to be executed when the condition is met}else {code to be executed when the condition is not met} Example:A program that gives the incorrect warning if the value entered from the outside is 1. $Alpay = "1"; if($Alpay == "1") { echo 'The entered value is correct.'; } else { echo 'The entered value is incorrect.'; }