The function call is not valid, which will result in a fatal runtime error.

This issue will be raised for any of the following cases:

The function is called with a different number of parameters than required.
The function is called with parameters of types different than expected.
Passing an expression to a parameter that requires a variable reference. Only variables can be passed by reference.

Please refer to the occurrence message for each instance to understand how it could be fixed.

function sum(int $a, int $b) {
    return $a + $b;
}

sum(1); // invalid: the function is invoked with 1 argument, 2 were expected

function sum(int $a, int $b) {
    return $a + $b;
}

sum('1', 2); // invalid: argument #1 ($a) must be of type int, string given

function sum(int $a, int $b) {
    return $a + $b;
}

sum(a: 1); // invalid: the function is invoked with 1 argument, 2 were expected

function incre(int &$a) {
    $a++;
}

incre(1); // invalid: a reference was expected, but an expression was passed instead

The number and types of parameters passed in must match the number of required parameters the function expects:

function sum(int $a, int $b) {
    return $a + $b;
}

sum(1, 2);

Pass in all required parameters when using the named parameter syntax:

function sum(int $a, int $b) {
    return $a + $b;
}

sum(a: 1, b: 2);

If a function marks its parameters as references, only variables must be passed to it:

function incre(int &$a) {
    $a++;
}

$a = 1;
incre($a);

References

EotvosCollegium / mars