The opcode is not the handler
The same loop ran 23% faster with int parameters, and only with OPcache on. Operand classes pick a handler when the file compiles; proven types pick a different one.
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Posts and notes.
Two tools printed different opcodes for one function
phpdbg showed thirteen instructions and the optimizer showed nine. Both were right. How to read the notation, and which listing is the one that runs.
Appending to a string is free until someone else holds it
The same .= statement took 0.269 ms and 218.836 ms in two loops that differ by one assignment. What a zend_string costs, where literals live, and why implode is not the fix.
A property hook emits the same opcode as a plain property
Plain, backed and virtual property reads compile to one identical FETCH_OBJ_R. What separates them is a function call the opcode does not show, measured at nine times.
A declared property is cheaper than an array key
One hundred thousand four-field records cost 143.54 bytes each as objects and 397.14 as associative arrays. Where the crossover is, and what one undeclared property costs.
Two hundred thousand objects, and the collector never ran
Two hundred thousand objects created and dropped, and zero collections. What puts an object in the root buffer, and why the collector's cost is set by what you keep alive.
Three numbers for one process, and memory_limit reads the middle one
memory_get_usage(), memory_get_usage(true) and ps disagree about one process. What each counts, which one memory_limit reads, and why a one-megabyte string costs two.
A by-reference foreach leaves a reference in every element
Walking an array with foreach ($rows as &$row) wraps every element in a zend_reference. On a million integers that is 32 MB the loop never gives back.
What a PHP variable actually is
A zval is sixteen bytes, a value union and a type tag. What that tag says decides whether an assignment copies anything, what a reference costs, and which variable pays.
PHP-FPM process management in production
pm static, dynamic and ondemand, the memory ceiling nothing enforces, and how to size a pool from measurements rather than guesses.