variable functions means that if a variable name has parentheses appended to it, PHP will look for a function with the same name as whatever the variable evaluates to, and will attempt to execute it.

function foo() {
echo "In foo()\n";
}

function bar($arg = '')
{
echo "In bar(); argument was '$arg'.\n";
}

function echoit($string)
{
echo $string;
}

$func = 'foo';
$func(); // This calls foo()

$func = 'bar';
$func('test'); // This calls bar()

$func = 'echoit';
$func('test'); // This calls echoit()

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