Integration tests are tests of the integration of different components that have been tested individually. Integration is expensive and it shows from the tests. You need to include time for this in your estimates and planning.
Ideally, you should organize a project so that there is not a phase at the end where integration explicitly has to take place. It is much better to gradually integrate the elements as they are completed during the project. If it is unavoidable, estimate carefully.
Some languages, which are formally defined syntactic systems, are not programming languages, but communication languages. They are specifically designed to facilitate communication through standardization. The most important are UML, XML and SQL. You should be familiar with them so that you can communicate well and decide when to use them.
UML is a rich formal system for making diagrams representing design. Its beauty is that it is both visual and formal, able to convey a lot of information if the author and the audience are familiar with UML. You have to know him because sometimes designs are communicated through him. There are some very useful tools for making UML drawings that look very professional. In many situations UML is too formal, and I find myself using a simpler style of boxes and arrows for design drawings. But I still believe that UML is at least as good for you as studying Latin.
XML is a standard for defining new standards. It is not a solution to data exchange problems, although it is sometimes presented as such. Rather, it is a welcome automation of the most boring part of data exchange, which is structuring the representation into a linear sequence and parsing it into a structure. It provides good type and correction checks, although again this is a fraction of what you will probably need in practice.
SQL is a very powerful and rich query and data manipulation language for php developers, which is not quite a programming language. It has many variants, usually very dependent on the product, which are less important than the standardized kernel. SQL is the “lingua franca” of relational databases. You may or may not work in any area that can benefit from an understanding of relational databases, but you should have a basic understanding of it, as well as the syntax and meaning of SQL.