Zend Technologies, the Cupertino start-up that sells software to develop Web sites based on the popular PHP scripting language, has raised $20 million in a fourth round of venture capital (release here).

By raising so much cash, the company must be aiming to produce a substantial return to make it worthwhile for its investors. It had already raised $16 million.

PHP is hot right now. Zend supports open source PHP initiatives, but it also offers commercial PHP products for software developers, and counts companies like gaming company Amp’d and social networking site Tagged among its customers. PHP does have is competitors. See our background on Zend and PHP here. Still, the company says PHP is the language of choice behind more than half of all Ajax-enabled site (Ajax being the highly interactive technology used, for example, in Google Maps).

This may also be a sign the company plans to go it alone for some time, at least if you believe the announcements by the company’s execs. Earlier this year, we reported that Oracle was in talks to acquire the company (scroll down). But now co-founder Andi Gutmans says he’s considering an IPO (Venturewire, sub required).

The round was led by Greylock Partners. Existing investors — Azure Capital Partners, Index Ventures, Intel Capital, Platinum Venture Capital, SAP Ventures and Walden Israel Venture Capital — also participated.

Correction: An earlier version of our post said Digg and Facebook were customers, something we’d seen referenced in the VentureWire piece cited above, and which do use PHP. However, turns out they are not Zend customers

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