What is the meaning of φ?

The golden ratio, an irrational number with a value of approximately 1.618033988 which expresses the relationship that the sum of two quantities is to the larger quantity as the larger is to the smaller.

Euler's totient function, an arithmetic function that counts totatives

a voiceless labiodental fricative (IPA [ɸ]).

Lower-case phi (φεῖ), the 21st letter of the ancient Greek alphabet. It represented the voiceless aspirated bilabial plosive /pʰ/ and later the voiceless labiodental fricative /f/. It is preceded by υ and followed by χ.

The lower case letter phi (φι), the 21st letter of the modern Greek alphabet.

Source: wiktionary.org