php - how to fix deprecated create_function error

i got this error:

Function create_function() is deprecated in shipping.php

private function calculate_string( $mathString ) {
    $mathString = trim($mathString);     // trim white spaces
    $mathString = preg_replace ('[^0-9\+-\*\/\(\) ]', '', $mathString);    // remove any non-numbers chars; exception for math operators
    $compute = create_function("", "return (" . html_entity_decode($mathString) . ");" );
    return 0 + $compute();
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as far as i found out,create_function is useless now. I considered all the suggestions on the web and came to the following conclusion

private function calculate_string( $mathString ) {
    $mathString = trim($mathString);     // trim white spaces
    $mathString = preg_replace ('[^0-9\+-\*\/\(\) ]', '', $mathString);    // remove any non-numbers chars; exception for math operators
    $compute = function() {
      return (" . html_entity_decode($mathString) . ");
    };
    return 0 + $compute();
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but this time it gives this error:

Undefined variable: mathString in shipping.php on line 1592 Warning: A non-numeric value encountered in shipping.php

Also i tried return part like this ways but it is not working

      return html_entity_decode($mathString);
      return $html_entity_decode($mathString);
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Where am I going wrong? Can you help me please?

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