PHP Memory Debugging

For one off my projects I need to import a very huge text file ( ~ 950MB ). I'm using Symfony2 & Doctrine 2 for my project.

My problem is that I get errors like:

Fatal error: Allowed memory size of 33554432 bytes exhausted (tried to allocate 24 bytes)

The error even occurs if I increase the memory limit to 1GB.

I tried to analyze the problem by using XDebug and KCacheGrind ( as part of PHPEdit ), but I don't really understand the values :(

I'am looking for a tool or a method (Quick & Simple due to the fact that I don't have much time) to find out why memory is allocated and not freed again.

Edit

To clear some things up here is my code:

$handle = fopen($geonameBasePath . 'allCountries.txt','r');

        $i = 0;
        $batchSize = 100;

        if($handle) {
            while (($buffer = fgets($handle,16384)) !== false) {

                if( $buffer[0] == '#') //skip comments
                    continue;
                //split parts
                $parts = explode("\t",$buffer);


                if( $parts[6] != 'P')
                    continue;

                if( $i%$batchSize == 0 )    {
                    echo 'Flush & Clear' . PHP_EOL;
                    $em->flush();
                    $em->clear();
                }

                $entity = $em->getRepository('MyApplicationBundle:City')->findOneByGeonameId( $parts[0] );
                if( $entity !== null)   {
                    $i++;
                    continue;
                }

                //create city object
                $city = new City();

                $city->setGeonameId( $parts[0] );
                $city->setName( $parts[1] );
                $city->setInternationalName( $parts[2] );
                $city->setLatitude($parts[4] );
                $city->setLongitude( $parts[5] );
                $city->setCountry( $em->getRepository('MyApplicationBundle:Country')->findOneByIsoCode( $parts[8] ) );

                $em->persist($city);

                unset($city);
                unset($entity);
                unset($parts);
                unset($buffer);

                echo $i . PHP_EOL;


                $i++;
            }
        }

        fclose($handle);

Things I have tried, but nothing helped:

1. Adding second parameter to fgets
2. Increasing memory_limit
3. Unsetting vars

Increasing memory limit is not going to be enough. When importing files like that, you buffer the reading.

$f = fopen('yourfile');
while ($data = fread($f, '4096') != 0) {
    // Do your stuff using the read $data
}
fclose($f);

Update :

When working with an ORM, you have to understand that nothing is actually inserted in the database until the flush call. Meaning all those objects are stored by the ORM tagged as "to be inserted". Only when the flush call is made, the ORM will check the collection and start inserting.

Solution 1 : Flush often. And clear.

Solution 2 : Don't use the ORM. Go for plain SQL command. They will take up far less memory than the object + ORM solution.