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.\'; }