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What is the difference between a language construct and a “built-in” function in PHP?

I have read that in php programming book "Language construct such asecho()andisset()can not be called through variable function"
What is the meaning of it ?

Answer 1

echo() and isset() (just to pick those exemplary for other PHP language constructs) can’t be called in a variable function.

Here comes an example of a variable function.

function foo() {
    echo "foo";
}

$func1 = 'foo';
$func1();        // "foo" will be output

And now let’s try with echo:

$func2 = 'echo';
$func2();        // "Fatal error: Call to undefined function echo() on line 10"

That’s because echo() isn’t a function, but a language construct.