How can I verify in PHP whether a variable both exists and has a value that is equal to a certain value?

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You can verify whether a variable exists and has a certain value in PHP using the following code:

if (isset($variable) && $variable == 'certain value') {
    // Do something
}

This code first checks whether the variable exists using the isset() function, and then checks whether its value is equal to the "certain value" using the == comparison operator. If both conditions are true, the code inside the if statement will execute.