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I what to add some data from some calculation. But I don't know how to implement the code.

<?php   
        $xx=$studenti;
        $yy=$promovare;
        $zz=$restantieri;
        $procente_promovare=yy*100/xx;
        $procente_restante=100-procente_promovare;
    ?>
<script>
var ctx = document.getElementById('myChart');
var myChart = new Chart(ctx, {
    type: 'pie',
    data: {
        labels: [' [%] Promovare', ' [%] Restantieri'],
        datasets: [{   
            data: [12,32],
            backgroundColor: [
                'rgba(255, 99, 132, 0.2)',
                'rgba(54, 162, 235, 0.2)'
            ],
            borderColor: [
                'rgba(255, 99, 132, 1)',
                'rgba(54, 162, 235, 1)'
            ],
            borderWidth: 1
        }]
    },
});
</script>

I want to add $procente_promovare instead of 12 and $procente_restante instead of 32. But I don't know how to add.

Modify the PHP block like this :

<?php   
    $xx=$studenti;
    $yy=$promovare;
    $zz=$restantieri;
    $procente_promovare=$yy*100/$xx;
    $procente_restante=100-$procente_promovare;
?>

You've missed several $ in there. And make the script to print the values :

data: [<?php echo $procente_promovare ?>,<?php echo $procente_restante ?>],