What potential issues can arise when using functions in PHP that rely on global variables?

When using functions in PHP that rely on global variables, potential issues can arise due to the lack of encapsulation and potential for unintended variable modifications. To solve this, you can pass the necessary variables as parameters to the function instead of relying on global scope.

// Potential issue with global variables
$globalVar = 10;

function multiplyByGlobalVar($num) {
    global $globalVar;
    return $num * $globalVar;
}

echo multiplyByGlobalVar(5); // Output: 50

// Solution: Pass necessary variables as parameters
$globalVar = 10;

function multiplyByVariable($num, $var) {
    return $num * $var;
}

echo multiplyByVariable(5, $globalVar); // Output: 50