Model/CountryInfo.php
public function withPopulation(string $population = null): self
    {
        $new = clone $this;
        $new->population = null === $population ? null : (int) $population;

        return $new;
    }
public function withPopulation(string $population = null): self
    {
        $new = clone $this;
        $new->population = null === $population ? null : (int) $population;

        return $new;
    }
public function withPopulation ( string $ population = null ) : self { $ new = clone $ this ; $ new -> population = null === $ population ? null : ( int ) $ population ; return $ new ; }
@param int|null $population

@return CountryInfo
@param int|null $population