What is tab injection?

Reading on the zend framework coding standard I found this phrase:

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Editors should be configured to treat tabs as spaces in order to prevent injection of tab characters into the source code.

And my question is: what is tab injection? I searched google but I did find this phrase in a lot of standards, so I 'm not really sure what it refers to.

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It's nothing special, just a fancy way of saying "we don't want tabs in our source code".

As stated on the link you provide, Zend coding standards mandate that whitespace in source files must not include tabs -- all indentation must be done with spaces.

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"Tab injection" means the insertion of a tab as an indentation character.