What is PHP's memory management and how to optimize it?

PHP allocates memory from the OS using its own memory manager, which maintains pools of pre-allocated blocks for efficient allocation. The default memory limit is 128MB (memory_limit in php.ini). Check current usage: memory_get_usage(). Optimization techniques: unset variables when done (unset($largeArray)), use generators instead of building large arrays in memory, process large datasets in chunks rather than loading everything, use streams for large file processing, avoid keeping database result sets in memory (use cursors), enable OPcache to reduce repeated parsing overhead. For CLI scripts processing millions of records, consider disabling the query log in ORM and batching operations. Memory profiling tools: Xdebug memory profiler, Blackfire.io.