Use str_contains over strpos for tutorial example

[timacdonald]

The first example on the PHP documentation that isn't a "hello world" is searching the User-Agent for the string "Firefox".

This example currently uses strpos. I believe we should be using str_contains.

I believe this for a few reasons, but to start with I'll let the function descriptions do the talking:

Find the position of the first occurrence of a substring in a string

vs.

Determine if a string contains a given substring

str_contains matches the semantics of what we are doing and eases the overhead of understanding what is happening.

When using strpos we are also introducing the reader to:

1. "Truthiness"
2. Boolean literals
3. PHP's "return false as a flag to indicate something" behaviour

Which is all additional overhead.

The current example also has double negatives that are hare to read left to right. The str_contains example feels much easier to comprehend, i.e.,

If strpos is not equal to false

versus

If str_contains

and...

returned non-false

vs

returned true

I understand that str_contain is a "newer" PHP function, however these are the documentation for the current PHP language - so we should take advantage and promote it's newer features, when it makes sense.

The str_contains function is available in all currently supported PHP versions.

Lastly, I removed the <br /> from the example as it serves no purpose to introducing the reader to "control structures and functions". The next example handles mixing in HTML in the "Mixing both HTML and PHP modes". The later examples also don't reference the <br /> anyway, so kinda feels like unneeded noise we ca drop.

[timacdonald]

The use of "must have" feels like there is a possibility that we are here and it didn't return true or non-false.

Question: "Did Frank clean the car?"

Answer: "Must have".

Narrator: "It was actually the rain the cleaned the car".

[TimWolla]

I understand that str_contain is a "newer" PHP function, however these are the documentation for the current PHP language - so we should take advantage and promote it's newer features, when it makes sense.

Agreed. This change LGTM, thank you!