Last modified time via filemtime($filepath) in php

Question

I am trying to test if write function write the log. The problem is the function don't return anything on success. I tried to find the accomplishment through finding out the last modified time. This is what i have scripted.

     /** Log file shows the following entries
     *8844 Nov-27 15:41:27 [5356] [0;31m[ERROR] Checking Error Logging...!![0m
     *8845 Nov-27 15:41:28 [4492] [0;31m[ERROR] Checking Error Logging...!![0m
     * 
     * */       

    $errorText = "Checking Error Logging...!!";
    $stub = $this->getMockForAbstractClass('Log');
    $stub->expects($this->any())
    ->method('Error');

    $BW_lastModified =  filemtime(LOG_TEXT_FILE);

    $this->assertEmpty($stub->Error($errorText));
    $AW_lastModified = filemtime(LOG_TEXT_FILE);

    $this->assertGreaterThan($BW_lastModified, $AW_lastModified);

the problem is filemtime(LOG_TEXT_FILE) returns the same time. I even tried to sleep the execution for 10 seconds by writing sleep(10) just before calling the Error function to write/modify.

any help would be really appreciated.