strcmp() not working properly, won't validate my data properly [duplicate]

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• PHP - String comparison is not working 3 answers

I have written some PHP to convert Fahrenheit to Celsius and vice-versa. I created the following function to validate the data and give an output:

function convert($convertNum, $string){

if (strcmp(strtoupper($string), 'F')) {
    $convertedC = ($convertNum * 1.8) + 32;
    echo '<h3>' . $convertNum . ' °C is ' . $convertedC . ' °F <br />';
}
else if (strcmp(strtoupper($string), 'C')) {
    $convertedF = ($convertNum - 32) * 0.8;
    echo '<h3>' . $convertNum . ' °F is ' . $convertedF . ' °C <br />';
}else {
    echo "The data you entered is invalid.";
}

}

However for some reason enter a string such as '32' and 'Z' still returns a value

32 °C is 89.6 °F

instead of outputting

The data you entered is invalid.