Opcode
Opcodes, short for operation codes, refer to low-level instructions that the PHP interpreter uses to execute PHP scripts. When PHP processes a code, it goes through several stages of compilation and interpretation. One of these stages involves translating the human-readable PHP source into a series of opcodes, which are then executed by the PHP engine.
Generally speaking, coders never come close to opcodes. The only occasion is when configuring the opcode cache: this cache keeps the generated opcodes in memory, and skips the stages of compilation as long as the source is not changed.