What is a language construct? How (theoreticaly) it is being implemented in C?

I am familiar with how PHP functions are mapped to functions in C in the C code beneath PHP. I know in C what a function means and what a MACRO() means.

I do not understand what is a language construct开发者_运维技巧, like echo, in PHP means.

"Language constructs" are operations that are supported by special features in the language. In PHP, mostly anything that isn't a variable or a function is a language construct, as referred on the list of keywords page.

As a PHP user, you have no way of implementing new language constructs. Those exist only at the language parser level. It seems your objectives are similar to those of this question, and the answer will be the same.