Standard libc process control (fork/exec/etc)
The libc fork() and exec() functions are not present in PHP by default. This is especially weird because the PHP functions would need only to be thin wrappers around the C functions, which would take very little space while substantially helping anyone doing systems programming in PHP (which some people do!).
The php popen function does not provide a pid.
They are provided by the PCNTL extension as php pcntl_exec and php pcntl_fork, but these are not even installed/available by default.
Significance: Missing Features
Language features make developers' lives easier. Often, language features are not complex for the language designers to implement (barring unnecessary complications in the internals), but can save developers hours of time. Missing language features are disrespectful to developers and encourage dirty hacks, "clever" solutions, and kludgy workarounds to achieve the desired functionality.