Per request the other side of the argument from @thexacre's answer.
IDEs strengths and weaknesses
So you go with multiple IDEs as no IDE is perfect and you devs each are more effective in their choice IDE. You now have people developing in PHP Storm, Notepad ++, Joes PHP IDE. As mentioned each of these applications have their strengths and weaknesses, and in some cases will create a nice synergy where when one IDE falls short, the other one picks up the slack...
Unfortunately that's a bit optimistic just as often as one IDE's strength will overcome another's weakness an IDE will introduce a weakness that wouldn't otherwise be there. For example PHP storm when your write a database call it uses a really efficient method of doing so by default, but Joe's PHP IDE really lacks in this area... If something is written in Joe's PHP IDE opening it in PHP Storm won't rewrite it. This means by using Joe's PHP IDE your application now has it's weaknesses in addition to PHP Storms!
Code standard woes
In even the most rigidly structured languages code standards become hot issues where certain things just never mesh. Now you move to code that's open source where these standards become far more lax. Luckily IDEs often help us adhere to some level of standard if for no other reason than the IDE defaults to it.
Now enter multiple IDES... Let's say PHP Storm organizes your code in a certain manner. As long as you stay true to this structure and/or use only PHP Storm navigating your code is easy! new we introduce multiple IDEs... Each has their own nuances on how they like to structure things. How are my conditions formatted? perhaps PHP Storm likes to put backend code into one place, while Joe's PHP IDE likes to put them in another.
Very quickly this can turn clean elegant code into the stuff nightmares are made of. Where every time you go to look for something you need either already know where it is, or you have to start going step by step to figure out where on earth it lives. Or you have 3 places to check for one thing... IE it's TERRIBLE!!! You can enforce this by being mindful, but expect to start that battle over every time you hire someone new on.
Tabs or spaces? (is but the beginning) Every IDE has it's own preferences on how code should be formatted. Some of these minor differences are extremely mild, others will bring about near holy war dispute among peers. In some cases the IDE won't even give you a choice it'll just reformat everything based on how it believes is correct.
Having the gap on an indent vary between 3 and 4 spaces is only the beginning. These things can change how you structure your code, when new lines start, whether you use single quotes or double quotes, etc. This stuff won't break code, but could break moral is developers are continuously faced with code formatting that changes hourly.
Source Control and Build Servers
In many cases most modern IDEs will play nice with any big name source control and build server solution. Though some will require plugins, addons, extensions, whatever...
However, if you are NOT using big name tools you might be treading into dangerous waters. PHP Storm might not support the solution you've chosen, or perhaps not well... But maybe that solution is excellent on Joe's PHP IDE... Do you now change tools to something that's not as suited for your needs to make sure both IDEs support it?
There are certain things terminology wise that are pretty universal in code. What is a method, class, etc. There are other things terminology varies. While most of the terminology should be the same confusion might come to pass where in one IDE a word refers to something that means something totally different in another IDE.
For example in one IDE "Addon" refers to additional functionality to allow use of third party tools, while in another IDE it's simply extending the functionality already in the IDE itself. (Like accessing a build server vs turning on unit testing)
Admittedly this isn't a huge issue, but it's something else to consider.
There are valid arguments on both sides. Ultimately you're choosing between the effectiveness of the individual vs the team. If everyone is 20% more effective using the IDE of your choice at the cost of only 10% effectiveness of the team using their own IDEs is a win! on the other hand if each individual is only 10% more effective at a 20% cost to the team then choosing one IDE is likely better.
And where your company falls depends on the culture, the work, the individuals, etc. It's something any metric you pull will be somewhat subjective or flawed, but you might be able to get a reasonably guess.
My gut feeling would be if there is a lot of pair programming or multiple people working on the same part of the application you're probably better off with just one IDE. If for the most part the devs work on separate things and mostly just come together at the end to merge or reference code allowing the devs to use their own IDEs might be better.