## Introduction to PHP and DTrace

DTrace is an always-available, low overhead, tracing framework available on a number of platforms including Solaris, macOS, Oracle Linux and BSD. DTrace can trace operating system behavior and user program execution. It can display argument values and be used to infer performance statistics. Probes are monitored by user created scripts written in the DTrace D scripting language. This allows efficient analysis of data points.

PHP probes that are not being actively monitored by a user's DTrace D script do not contain instrumented code so there is no performance degradation during normal application execution. Probes that are being monitored incur an overhead low enough to generally allow DTrace monitoring on live production systems.

PHP incorporates "User-level Statically Defined Tracing" (USDT) probes that are triggered at runtime. For example, when a D script is monitoring PHP's `function-entry` probe, then, every time a PHP script function is called, this probe is fired and the associated D script action code is executed. This action code could, for example, print probe arguments such as the source file location of the PHP function. Or the action could aggregate data such as the number of times each function is called.

Only the PHP USDT probes are described here. Refer to external general and operating system-specific DTrace literature to see how DTrace can be used to trace arbitrary functions, and how it can be used to trace operating system behavior. Note not all DTrace features are available in all DTrace implementations.

The static DTrace probes in PHP can alternatively be used with the SystemTap facility on some Linux distributions.

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