Can police detectives use Iphone to record evidence?

Can police use an iphone to record a witness statement or take pictures for use in an investigation

It is possible, but highly unlikely.

Everything that an officer records can be used as evidence. There must be a clear chain of custody to book in the evidence and to ensure that no evidence is deleted or missing. The iPhone does not offer these features. I imagine in an emergency perhaps an officer might choose to use the iphone to record evidence. After the evidence is recorded the entire ip[hone would have to be submitted to an evidence custodian.

Yes.A camera is a camera.

Yes.
There are two primary types of photographs that are used as evidence - "category 1", where your testimony is the evidence and the photographs are used merely to illustrate the testimony that you are providing, and "category 2", where the photographs themselves are the evidence and are usually used for a scientific analysis [close-up photographs of fingerprints, shoe or tire impressions, tool-marks, etc]. The requirements for a "category 1" image are much less stringent than "category 2". Lossy formats such as JPEG are common when it doesn't matter if the camera averages out a color value for some of the individual pixels. [A tree is still obviously a tree even if every leaf isn't the precise shade of green. The human eye can only distinguish 256 different shades of gray anyways.]
I have seen many Officers packaging evidence items to be submitted to their Property Unit who will take a quick snapshot of an item they are submitting. They will then preserve the photo file in accordance with their department's policies to help them recall the case years later when it goes to court. The digital file with the embedded metadata and appropriate custody trail is usually sufficient - they do not need to hand over their personal phones without a specific reason to do so.
While an iphone is not the best method of recording witness statements, if it is all that is available, there's no reason not to use it. Usually a juror would rather hear a bad recording to supplement the Officer's testimony than have no recording at all. In order to use the recording, it would need to be properly authenticated first.