AbstractingTime.php

<?php

final class UselessUnixTimestampProvider
{
  private $clock;
  
  public function __construct(Clock $clock)
  {
    $this->clock = $clock;
  }
  
  public function timestamp(): int
  {
    return $this->clock->timestamp();
  }
}

interface Clock
{
  public functiom timestamp(): int;
}

final class SystemClock implements Clock
{
  public function timestamp(): int
  {
    return time();
  }
}

final class FixedClock implements Clock
{
  public function timestamp(): int
  {
    return 1483615054;
  }
}

// In tests
$provider = new UselessUnixTimestampProvider(new FixedClock());

// In production
$provider = new UselessUnixTimestampProvider(new SystemClock());