Mysql – What are the consequences if I backup LIVE databases every 3-hours using thesqldump

Full backup contains both the data and log. For data, it simply copies each page of the database into the backup, as is at the moment it reads the page. It then appends into the backup media all the 'relevant' log. This includes, at the very least, all the log between the LSN at the start of the backup operation and the LSN at the end of the backup operation. In reality there is more log usually, as it has to include all active transactions at the start of backup and log needed by replication. See Debunking a couple of myths around full database backups.

When the database is restored, all the data pages are copied out into the database files, then all the log pages are copied out into the log file(s). The database is inconsistent at this moment, since it contains data page images that may be out of sync with one another. But now a normal recovery is run. Since the log contains all the log during the backup, at the end of the recovery the database is consistent.

1. If you are just doing dumping, gzipping and then sending email, there is no difference. BUT if you are doing more complicated stuffs, using PHP will have advantage, example maybe you want to set timestamps, or parse the output before sending. Then you can make use of PHP's vast libraries and methods for these kind of stuff that batch can't provide (at least easily)

2. This one you will have to test out yourself.

3. You might want to set up incremental backup or save just the transactions.(not the whole database)