On the Snyk.io site they've announced the release of their State of Open Source Security Report for 2019. In this report they talk about packages and managers outside of the PHP ecosystem, but there's also plenty in there about general Open Source security, regardless of the technology used.

We’ve seen big technology players doubling-down on open source in 2018. In every registry we reviewed, we saw an increasing rate of open source libraries being indexed in every language ecosystem. This is to be expected, but the rate of growth may come as a surprise to many.

[...] In 2017 the CVE list reported more than 14,000 vulnerabilities, breaking the record for the most CVEs reported in a single year. 2018 continued the record-breaking streak with over 16,000 vulnerabilities reported.

We can see how open source package growth translates into user adoption when looking at the download numbers for various packages in different ecosystems.

They specifically cover packages in the Node.js, Python and Java worlds but the same principles apply to PHP and Composer packages too. There's a few other related posts that go into more detail on the vulnerability increases, the desire for Open Source developers to be security-minded and other topics. You can get all of the information in one place, though: the PDF version of the report.